people aren't inherently evil, and when they do something, they almost always believe it to be for the better 
Well that point has more to do with religion even. I don't think people are "iinherently evil" but I think they do things sometimes that they know deep down is wrong or evil but they try to JUSTIFY it so they don't have to feel bad about it. IF you think of Hitler I bet part of him knew that making people suffer was wrong. I mean the guy was raised catholic. That doesn't really go away even if you lose your faith totally. But he tried to justify it by saying he was at war with jews, at war with the world, working for the "survival" of his race and things like that.
I bet a few ppl who did slavery felt that some of it at least was wrong, that parts of the system was wrong. But they justified it in their mind by thinking it was best for all concerned. They justified it by thinking about profit or about the fact that they had a good relaionship with their slaves and treated them well or thinking that they were taking care of them. etc,,,
So its not the people themselves that were evil, But the system itself was evil and allowed ppl to feel good about doing something that in general was not.
So I am not saying we can always judge the people but we can judge the systems they put in especially if they put in something NEW (like race-based slavery) just so that they can keep having slaves and profit, etc...
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"Everyone helpin' each other whenever they can we makin' it happen, from nothin' to somethin'
That's how we be survivin'" - BEP