I think people start allowing themselves behaviors on the fringe, which then changes their perception of what is acceptable and further pushes their belief of w

I think people start allowing themselves behaviors on the fringe, which then changes their perception of what is acceptable and further pushes their belief of where the fringe is, which then gives them reason to allow themselves behaviors on that new fringe, and so on... Eventually they end up somewhere messed up. Morality is malleable and normality is relative. Unless you pay attention at every step, I can see how someone who's playing a bit loose with the rules might end up in a spot where they're doing truly awful things.