The history of mankind clearly shows again and again, man's inhumanity to man, of which, wars, selfishness, fear, and greed, bring out the worst in many a person and civilization, time and time again. The thing about war, in any of its myriad forms, is that most of the participants in warfare, don't really have an axe to grind with their opponent, and in absence of such a grievance, aren't particularly motivated to participate in a war, though, countries and their governments are usually highly skilled at providing such motivation, of which the primary one comes down to the demonization and the dehumanization of the enemy, of which, to acknowledge the enemy as being a fellow human being, of someone who is just as worthy of respect and consideration as one self, is not even an allowable thought.
This means, that in order for something like slavery to actually be permitted or countenanced, or to wantonly and to indiscriminately kill the opponent in war, the very first thing that must be accomplished, is the propaganda or proselytizing that makes it clear that the other is either not a true rational human being in the proper sense of the word, and therefore is something like a subspecies to a human being, or is quite frankly, not human at all, and rather more appropriately seen as vermin, rubbish, scum, diabolical, devilish, or as something that is for a certainty, god-forsaken, and not worthy of any sort of decency or respect, being accorded to it.
This then, makes it rather easy to indiscriminately kill your opponent, because the person doing the killing does not see the other as fully human or even considers that they may actually be human. It also makes it rather easy to take someone that is regarded as less than human, and then for humanitarian purposes, so to speak, train that captive as if they are an animal, and treat that captive as a beast of burden, of which, believing that the captive is an animal, the person that is in control, feeds, trains, and uses that captive as an animal, and will not consider in their thinking, anything that would have a strong tendency to upset that belief, because to even consider that somehow the captive is human or has human-like capabilities, would necessitate a re-evaluation as to whether what is occurring is actually right or wrong, for an animal, clearly is a species, that mankind has total dominion of, and thereby can do whatever that they so will to that animal.
Most people, like to consider themselves to be fair, and to believe that their actions, are therefore fair-minded, so too, most people clearly believe that murder and the taking of another human life is wrong, while recognizing that there are some notable exceptions to that rule; as well as most people, believe that to own another human being and therefore to be able to do whatever that it is that they wish to by virtue of that ownership of that human being, doesn't sound morally right. Yet, we are surrounded by armed conflicts, wars, and the enslavement or oppression of other human beings, virtually, all over this globe, indicating that most people, are just fine with the killing and the owning of others, just as long as they are able to believe, that those others aren't really human, or at least, aren't human enough.