The typical whitewashing goes something like this:
"Slavery" in the Bible should not be confused with what we commonly understand as slavery, ie. the kind of slavery that was prevalent for example in the southern United States in the past. In the United States slaves were basically like animals, often even less. They were regularly mistreated, abused, tortured and even killed, and of course they were slaves for life. "Slaves" in the Bible, however, were more like servants. They were well cared for, they could own property, and they could even buy their freedom. God's law imposed severe punishment on those who mistreated or killed their slaves. Slaves were also freed as a matter of course each seven years.This kind of whitewashing sounds good and grandiose to those believers desperate to have slavery in the Bible explained to them, and why it's not a bad thing there. With rationalizations like this they can shut the nagging voice in their head that casts doubts on this particular aspect of the Bible (which should otherwise be perfect.)
However, this rationalization (which is extremely common) ignores several facts, all of them found directly from the Bible itself. (There are also many other facts that we know from history.)
Firstly, this kind of rationalization glosses over the fact that many of said slaves were spoils of war. There are numerous examples in the Bible where, very directly and unambiguously, it is told how the people of Israel invaded an enemy city, killed all of its men, and took its women and children as spoil of war.
Most apologists dare to object to this by claiming that this was actually an act of mercy. So let's get this straight: Enemy combatants invade your city, kill your husband, father, brothers and other male family members, possibly before your very eyes, and then take you and your sisters and children against your will as slaves to a foreign country. And this is supposed to be an act of mercy? In which universe?
Most importantly though, regardless of how "well treated" these slaves might have been (which in itself is a suspect claim,) it's undeniable even from the Bible itself that they were considered property (which is directly stated as such,) and that they were not free to do whatever they wanted and leave as they wanted.
Owning other people as property is an abomination, and nowhere does the Bible forbid this practice. If the Bible were truly perfect and the word of a benevolent God, most certainly there would be clear prohibitions against this, along the lines of "you shall not own other people as your property, for that is an abomination in the eyes of your Lord."
Moreover, not only does the bible not forbid owning people as property, it actually does not forbid mistreating them either, unlike the typical rationalization claims. There is an infamous commandment in the old testament that if you hit your slave and he dies immediately, you must be punished, but if he dies a few days after, then no punishment. Curiously, and quite inexplicably, apologists take this very passage to demonstrate how God forbids mistreating of slaves, and how killing them is deserving of punishment. (They argue that the exception in that law exists because if the slave dies a few days later, it's not possible to tell if he died because of being hit or because of something else.)
These apologists are missing the main problem with this, and that's that the passage doesn't actually forbid hitting your slaves. It only forbids hitting them so hard that they will immediately die. In other words, according to the Bible, it's allowed to hit your slaves.
(By the way, it's quite incredible and outright amusing how much some apologists and Christians read between the lines and add their own embellishments to this particular passage in order to whitewash it. One of the worst ones I have seen claimed that it's actually talking about a soft rod used to discipline unruly children, and slaves, and that you were only allowed to hit once... because you know, the passage talks about hitting once, therefore we can deduce that hitting the slave twice is forbidden. It doesn't matter that nowhere does it say anything like this. But we can read it between the lines, of course.)
From history we know that there were several types of slaves in Israel; and in fact there are references to this in the Bible itself as well. The two major types of slaves were, as already said, foreigners taken as spoils of war, as well as indebted Israelites who paid their debts in the form of involuntary servitude. If we examine the passages talking about slaves buying their freedom, and being freed each seven years, it becomes quite clear that these passages are, in fact, talking about the indebted people who are paying their debts with servitude, not the ones taken as spoil of war.
The New Testament does nothing to correct any of this. The status of slavery is taken as a matter of fact, as a fact of life. Instructions are given in several passages to slaves (for example to be subservient to their masters.) Nowhere is it declared as an abhorrent practice nor prohibited.
As far as the Bible is concerned, slavery could well be practiced today without it being against any commandment or teaching in the Bible. Yet, curiously, no Christian endorses this. (And quite ironically, no Christian ever thinks where this new moral code came from. It most certainly did not come from the Bible.)