I respond to some objections that @AdamSkyThief raised regarding the Old Testament. Much of this revolves around the question of how to reconcile the supposed cruelty of the God of the Old Testament. We begin with Leviticus 25:39-46.
The first thing I would call to your attention here is that the word "slave" does not occur in this passage according to the King James Bible. The word "bondman" is used. I am not sure what the original Hebrew word for bondman is. I suspect that there is some difference in meaning from "slave" because the word "slave" is used elsewhere in the Old Testament (albeit rarely). Also, Hagar is referred to as a "bondwoman" in Genesis 21, however it is recorded later that an angel of God helped Hagar and her son, Ishmael. The word "slave" connotes someone who is viewed as expendable (e.g. the slaves who built the pyramids), but it seems that the word "bondman" or "bondwoman" connotes something more — as if there were an obligation on the part of the master (in Hagar's case, Abraham) that the master either does — or does not — fulfill. Again, I am not certain on this, but that is how it reads to me. For the purposes of my response, I will use the term "bondman" because the word "slave" is too bound up in the West's own history with slavery and carries too much baggage.
A few things to note about the passage you cite. Israelites are not to treat fellow Israelites ruthlessly, and this you know. But there doesn't seem to be an element of coercion in this passage: "Moreover of the children of the strangers that do sojourn among you, of them shall ye buy, and of their families that are with you, which they begat in your land: and they shall be your possession." The writer here says, "of them shall ye buy." That means that some — perhaps even many — of these strangers, these sojourners, were not bought but remained free. So there seems to have been an element of voluntary cooperation here. Perhaps as a sojourner I would sell my labor as Jacob did and then return to my family, or marry a wife, or what have you. But it was not an arrangement into which I would be forced. It is an arrangement I would enter into voluntarily. Also, there is no element of coercion on the part of the master. Yes, it does say that "they shall be your bondmen forever" but there was always the option of letting the bondman go. In fact every Jubilee — every 50 years — all bondmen had to be let go to return to their families. Finally there is the sense in which these bondmen become brethren over time (verse 46), because otherwise it wouldn't make sense for the writer to say the same thing twice ("ye shall not rule one over another with rigour.").
Another argument in favor of better treatment of slaves is the commandment "Thou shalt not steal." Yes this commandment applies to things, but much more importantly, and as @cmurchada pointed out, it applies to people too. Kidnapping is forbidden. You cannot steal a human being. And that is precisely what slavery — as opposed to indentured servitude or the relationship between a bondman and his lord — aims to do. Slavery steals the human being and his labor. If bondmen were truly slaves, then the Israelites would have been violating the commandment "Thou shalt not steal."
Finally, as I alluded to in my article, Slavery and the Bible, the primary concern of the Biblical writers is that bondmen (I will use this term to include bondmen and bondwomen) were to be treated justly. There was no "beating a slave within an inch of his life." If the master murdered a bondman he could be put to death (Genesis 9:6). In order to ensure the just treatment of bondmen, they participated in the Sabbath, they were to be treated humanely, and they were freed each Jubilee. There are probably other provisions, but let me deal with some more of your page here.
You noted that "every regulation...has the caveat that it applies only to Israelites." That may or may not be true. As I hinted above, I don't necessary read all of these regulations that way. But let us assume you are correct. I would respond by saying that there are higher set of "regulations" called the Ten Commandments, that apply not just to Israelites, but to everyone in all times and places. But the Israelites were required to observe these commandments. That means that the Israelites could not murder, steal from, commit adultery with, covet the possessions of, or bear false witness against, a bondman. And that's just five of the Commandments. That's a pretty significant set of protections for bondmen, that is, for the people you refer to as slaves. In fact, combined with an "eye for an eye," and some other O.T. laws, bondmen enjoyed a set of rights and protections in ancient Israel that were unparalleled in the ancient world. Just compare a bondman in Israel to slave in Egypt and you'll get the idea.
So when you say, "God doesn't have any reservations about slavery," I think we have to take that with a grain of salt. What God permitted in the Old Testament was the voluntary arrangement whereby one person sold his labor to another person. The former became a bondman and the latter his lord (or master). But just as the bondman had obligations to his lord, so the lord had obligations to his bondman. And, Hebrew society as a whole protected that bondman with many of the same protections free Israelites enjoyed. I would argue, on the contrary, that God disapproved of slavery, and that the arrangement between bondman and lord was not a "slave/master" type of relationship. The bondman certainly was neither anonymous nor expendable. From the point of view of the Pharaoh, the Hebrew slaves in Egypt were both anonymous and expendable. And that is why God liberated the Hebrews from the land of Egypt and overthrew the Pharaoh and his army.
Next, you claim that God "commanded Moses to slaughter the Midianite people and take the virgin girls for themselves." While this may be prima facie accurate, nowhere in the passage does the OT writer condone sexual slavery. That is our contemporary reading of the phrase "take girls for themselves," but remember that this is a translation from an ancient language (either Greek or Hebrew depending on whether you are reading the Septuagint or Masoretic texts); therefore, we have not proven that "sexual slavery" is what the author meant 3,000 years ago. Again, we have to be careful about superimposing a modern reading of terms and language onto the Biblical text and ancient civilizations.
Next, the claim that because Jesus is God, Jesus must have supported biblical slavery and sex slavery. I dealt with these specific items above, so let me reformulate your statement thus: because Jesus is God, He must have supported any action that God did in the Old Testament that we perceive as wrong or evil — as if Jesus were somehow different from the God of the Old Testament. But Jesus is God, period. He is, specifically, the second Person of the Trinity. St. Paul tells us in the letter to the Hebrews that He is the very Word of the Father, "the Brightness of His Glory, the Express Image of His Person." Jesus is "He by Whom God [the Father] made the ages," the very Logos and Wisdom of God. So if we deal with objections to the Old Testament God, we have dealt with objections to the New Testament God, for it is written, "I and my Father are One" (John 10:30). To believe that the New Testament God and the Old Testament God are different gods, or to believe that the New Testament somehow invalidates and renders obsolete the Old Testament, is Marcionism, which was condemned as a heresy back in the 3rd century AD.
Regarding Exodus 21:20, the passage seems to connote the opposite of what you thought: "And if a man smite his servant, or his maid, with a rod, and he die under his hand; he shall be surely punished." When a human life is taken by intent or by reckless endangerment, it must be avenged. Thus, like any murderer, a master who murders his slave is subject to the death penalty...the Torah is unique in protecting slaves from homicide and other forms of maltreatment by their masters." -Dennis Prager, commentary on Exodus, "The Rational Bible: Exodus." Regarding verse 21, Prager does say this is troubling, and fortunately I have his commentary open in front of me. He writes, "as always, we must first go to the Hebrew original: "But if he stands for a day or two, he is not avenged..." So the Hebrew says "stands." This may imply that if the slave is not so injured as to be unable to get up, the master is not put to death some time later. In other words, if the slave (or bondman) is strong enough enough to get up and stand for a day or two, the master is not punished with the death penalty. Prager does NOT say that the master is not punished at all. He adds a number of ancillary points which I do not have time for right now.
Regarding breaking the Sabbath and the death penalty, the verse does not merely refer to the breaking of the Sabbath. It refers to the profaning of the Sabbath; that is, making that which is holy, unholy. The Sabbath is the primary means of expressing the most fundamental of all truths: that God created the world. The Israelite who profaned the Sabbath in public undermined the most fundamental teaching of the Pentateuch (or Torah). He was intentionally leading others to deny God created the world at the very time Moses was attempting to inculcate the basis of ethical monotheism (that there is one God and His primary demand is goodness, i.e. moral behavior). It was the element of publicity added to the violation that raised it to the level of the profane. Again, though, it must be remembered that Torah law — and later Talmudic law — regarding capital crimes generally essentially ensured that Sabbath violators were not executed.
Regarding the guy collecting sticks (Numbers 15:32-36), look at the two verses preceding (Numbers 15:30-31):
But the soul that doeth ought presumptuously, whether he be born in the land, or a stranger, the same reproacheth the LORD; and that soul shall be cut off from among his people. Because he hath despised the word of the LORD, and hath broken his commandment, that soul shall utterly be cut off; his iniquity shall be upon him.So again, this actually supports what I said above: that it was not the private violation of the Sabbath that got people into grave trouble, but the public violation — that I can flout the laws of God with impunity in front of all my brethren and get away with it. Sodom and Gomorrah tried that, so did Korah, and a number of other Biblical figures, and it didn't work out so well. This guy obviously didn't learn the lesson of Sodom and Gomorrah. What is most notable to me though is that God doesn't administer the death penalty here. He has the Israelites do it. God seems to have foreseen the argument that man should never take the life of another man. He declares this idea false in Genesis 9:6. I'd also be willing to wager that this man collecting sticks was the only person in recorded Jewish history to be put to death for profaning the Sabbath. As an aside, there is a tremendous moral difference between public and private behavior, but this is a topic for another time.
There is one particular Old Testament law which I would like to bring to your attention. That is the law that if a son strikes his parents, the son shall be put to death. This is often used as an argument of the supposed cruelty of the God of the Old Testament. However, like an "Eye for an Eye" and other laws, it is actually quite a brilliant achievement, in fact one of the most so. The purpose of this law was not so much to kill children who struck their parents. In fact, it was the opposite. In many cultures during this period (3,000 B.C.) ownership of children by their parents was absolute. We often joke to our kids when they misbehave that, "I brought you into this world, and I can take you out!" However, in those days, this was no joke. It was deadly serious. Parents had life-and-death authority over children. Now, it would have been simply unacceptable to have a law that prohibited parental punishment or control of children. But the power of life and death is removed from the parents to the system of Hebraic courts. The parents no longer have the right to unilaterally take the lives of their children, because the children are created in God's image. If an infraction is very serious, the parents can present their case before a court, and if the court decrees, the child would be put to death. There is no recorded case in Jewish history where this penalty was ever carried out. The genius of the law was that it preserves parental authority, it emphasizes the seriousness of disrespect to parents, and it communicates that children are created in the image of God, and their lives have intrinsic worth.
"Belial," which is mentioned in the passage from Deuteronomy that you cite, is another name for the Devil. Many of the contemporary civilizations, which had presumably existed for hundreds of years before the Israelites came on to the scene, were incredibly wicked — essentially devil-worshiping nations. How wicked were they? Well for example societies who worshiped Molech (another name for the Devil) burned infants and children alive as sacrifices to Molech. If that's not wicked enough, let me do more research and I'm sure I can find some other things. But the point is, in my opinion, for these civilizations, their number was up. The Judgment of God was upon them for the monstrous evil that they had done. The Israelites were assigned by God to carry out the divine judgment (no pleasant task). These people weren't just "some pagans," these were really bad people — we are talking about MS-13 levels of cruelty going on here! It would have been wholly justified, in fact perhaps morally obligatory, for any nation to go to war to stop such evil. If we found out that another country was burning its children alive and sacrificing them to fake gods, wouldn't we want to stop that? Any sane, moral reasonable person would — and God did.
Well, I have attempted to answer most of your points or objections. I hope this has helped. I don't have an answer to every question — I wish I did. But I think the primary lesson here is to remember there is always more to the story. We have to really dig into this stuff, and not judge it by a superficial reading of the text.