When you hear about someone you know, doing a bad thing, don't you feel kinda sad or maybe even guilty about it?
Because you think, "What was my part, among all others, in their life which lead them to this path?"
I just realized why people hate each other.
Once someone sees the worst side of another and continues to judge them by it, they eventually starts hating the latter.
Suppose the person A knows the worst mistake the person B has made in their life and judge their (B) every action based on the knowledge they (A) acquired, they (A) starts to hate them (B). In turn, the person B also tries to find the worst of the person A and starts to hate them as well.
This is how it goes on and none of them ever tries to find the person behind all that mistakes and weaknesses.