But why even bother discussing the concept of evil if we cannot acknowledge that there must be an absolute measure to some degree?

The reason I discuss this so vehemently is because of the revulsion I experience at the concept of slavery - I have a hard time understanding someone who doesn't feel the same way, because I don't understand how someone can't find it to be utterly monstrous.

What choice do I have but to condemn those that don't agree? This is not a matter for opinions in my mind; it's simply inconceivable to me that someone would feel otherwise about slavery. I have to conclude that anyone who disagrees either hasn't thought it through, or has some kind of deficiency that prevents them from empathising with other human beings (or, as I boiled it down to on the forum I left: "you're either stupid or insane. Feel free to pick whichever one you prefer").

It's like someone insisting the sky is red or something. I can't take that person seriously, and I simply assume their reasoning is flawed.

Agreed. Precisely my point through most of this discussion, and the reason that moral relativism is so fraught with difficulty. Some things are just hard to accept as cultural bias or opinions - as human beings, we're hard-wired to find some things fundamentally repellent.

I would argue that the widespread existence of slavery in past ages was due to the belief in most people that anyone who was demonstratably different from themselves (i.e. with different coloured skin, coming from somewhere else in the world, being a woman, not obeying certain arbitrary cultural norms) wasn't really thought of as a 'human' at all. This is a side-effect of the necessity for primitive human cultures to each have their own "make a human kit" that they used to produce functioning members of their own society through elaborate rituals and traditions. Enforcing such arbitrary conditions upon a member of their tribe (for example, reciting certain words or bearing certain ritual disfigurements - both of which currently mark out members of the Jewish faith even today) ensured they could never be infiltrated by an outsider and that their genetic lineage would survive.

Racism and, eventually, slavery, is the sad upshot of such important steps in human social evolution when people start multiplying and wandering around the globe.

Slavery was a terrible by-product of social and industrial "dominance". Cultures that felt they were "superior" to the ones they chose to exploit, felt that their choices were nothing more than pack animals.

It wasn't as if they even allowed these people basic human dignity. Slaves were property and treated in most cases worse than most farm animals.

If you can, fathom this. You are in your bed, in your house, whatever part of the world you are in. Someone breaks into your house, kills anyone who resists them, and shackles those who wimper but don't resist.

Next you are laid down in a row, in a dark room, with several of your fellow countrymen, shackled to the floor. For weeks you are kept in this position. Your savage captors scream at you in an undecipherable tongue. They beat anyone who cries out for food, help, anything. You get a cup of sea water and a cup of half-cooked rice to eat every week. You sleep in raw sewage. You defocate and urinate on yourself. You have no change of clothes. You are barely able to sit up. People around you vomit with various sickness and reaction to the environment.

When you are finally pulled out of the room, you are shackled to your weak, fellow countrymen, and herded like cattle. As you are lead to your next destination, anyone who falls over is beaten, some to death. Anyone who attempts to help them to their feet often suffers a similar fate. If any members of your family have survived, you are likely separated, with no hope of contact. You and your people seem to have been sorted into specific groups. You are herded to work in fields, mines, and many other tasks that involve hard labor. There is no pay. Your compensation is your existence.

Now imagine that this goes on for generations. People are able to communicate with their "masters". The fate of children is that they are born into the machination of slavery. It will be all most will know. Most will accomplish their tasks with complete resignation. The "masters" will beat and kill people for apparently no reason other than sport. The "masters" will rape the women, often in front of their male partners or family or their young children, while threatening them with their lives. People who try to end this madness often suffer death.

Now please explain to me what was not evil about slavery. Even the aftermath that reverberates to this very day, is nothing short of a steaming pile of dogshit.

Also that ^

I think you get why I wasn't in a hurry to hang around this other forum, Bam.