Slavery began so early it was already old when we started writing things on clay tablets. You might expect it was the cruel notion of some king but actually it was a small mercy of some warrior who decided to milk the cow, so to speak, rather than slaughter it. Prior to this it was the custom to serve war captives for dinner. After this it became the custom for them to serve dinner. The first slaves would of been women since they are less dangerous and more open to changing allegiances when treated right 1. But it's hard to kill a skilled craftsman who accurately critiques the knife you intend to do it with so some men began to be spared. After a while of this Slavery becomes a custom and an etiquette forms around it. The most important of which was a limited term. Since life long slaves have little to lose they are untrustworthy. But one with a term limited to 7 years has a reason to stay on good terms with the society they serve. At this point slaves are highly productive and safe to be around which in turn makes war more profitable. Slavery became a business.
This was a pre-christianity world so most people didn't see much wrong here. Might is right was self evident at the time. Even being a merchant or tradesman was looked down upon, they thought it was weak to serve others even when in exchange for money 2. There were of course plenty of philosophers who believed otherwise such as the Stoics, but their ideas had to wait for Christianity to reach the common man.
From the slaves perspective Christianity didn't change much. It just gave them a believe system to alleviate the pain of slavery. Their actual lives continued as before for another millennium. It's important to note at this point slavery has nothing to do with race 3. If you were a slave it just meant you lost a war recently. The racial aspect only came into it after the African slave trade had been operating for long enough that most slaves were black. This was the first time in history that there was a significant ethnic difference between slaves and free people. The Slavs could lose there accent in one generation and become indistinguishable from the general population but the Africans required at least 4 generations to do the same. This was the worst time to be a slave in history. Though it wasn't exactly as some people believe.
The worst part about being a slave in this time period is that your term was unlimited and your skin colour marked you out. However, almost all slave owners were Christians who believed in the equality of human souls. So they tended to treat their slaves with a certain level of respect. They also tended to care what their slaves thought of them so they would give them gifts and assign them just enough work to keep them busy into the early afternoon. Then allow them free time the rest of the day. Many slaves used this free time to grow fruit which they owned and could sell. If they did this enough they could eventually buy out their own title and become free. Because of your skin colour people would know you were a freedman and would therefore assume you were poorly educated and lacked generational wealth. You weren't actually prevented from acquiring these things though and some freedmen did go on to acquire generational wealth and even some white slaves. Though as time went on the racism got worse and this kind of thing would of become more difficult.
Fortunately, though it was also the time that the industrial revolution was making slavery less profitable. New machines such as the cotton gin were reducing the demand for drudgery. In addition Christians had grown to really dislike the practice. The Protestant English in particular who valued freedom and believed that god made men free and intended them to live free. It became known as the peculiar institution because while everyone preferred it would go away it persisted never the less 4. This is because in certain industries it remained more profitable. So anyone that tried to operate in those industries without using slaves would simply go out of business. Pretty much everyone in America was in favour of measures designed to prevent Slavery from growing. Straight up abolishment was viewed as impracticable for many good reasons. But thanks to some happy accidents Lincoln both came to power and won the civil war which enabled the end that most people wanted, to arrive sooner than was thought possible 5.
Similar to the way ennobling servitude had the unexpected long term affect of completely destroying the worst form of it. Abolishing all forms of slavery might have the long term affect of actually increasing the worst forms of it. This is because by removing the ability to own a human you remove a financial instrument which that person could of used to start a business. If a person can sell themselves then they can use their title as collateral on the loan they need to start a business. Without access to that loan they have no choice but to sell their labor and wait until they have accumulated the capital they could of had immediately with the loan. This will typically be around 7 years at current interest rates which is around the time most slavery terms lasted anyway. Hence by abolished the formal concept of slavery we have actually increased the amount of servitude that the best among us perform in our life times. So perhaps the optimal amount of slavery isn't zero if ones ideal is maximum freedom. The world is complicated.
The causation might run the other way. Perhaps women became quick to change allegiances because they were so often captured that any women that did otherwise would fail to reproduce at a high enough rate to stay in the gene pool
Perhaps an overlooked advantage that christianity provided was the belief that it's noble to serve others. Which eventually created a positive sum world of trade which was wealthy enough to end slavery. Kind of ironic that by ennobling servitude christianity ultimately reduced it. The world is complicated.
Though the word itself is derived from Slav, as in Russian. Because for a while there that region was a soft target and provided Europe with a lot of slaves. Before then they were known by various names. The Romans called them servants. The Greeks called them 2 footed cattle. Every language had it's own name.
A little like the animal farming industries today. Everyone agrees that there are aspects of farming that are probably very unpleasant for the animals involved it's impossible currently to destroy those practices because most consumers will only pay slightly more for the ethically grown stuff. I include myself in that and part of the reason is that I don't know if the ethical products are really more ethical, though that might be motivated reasoning on my part.
Americans prided themselves on living in the land of the free so naturally slavery bothered them as it was blatant hypocrisy. However, the British also hated slavery and acted upon this belief before the Americans were strong enough to do it themselves. This triggered some pettiness that probably extended slavery in America a little longer than it otherwise would of survived.