Why are people so crazy?

I guess some people have their limits. things happening to them and they get pushed off the edge, so to speak. it's very sad really. I wish people would do more to help each other out. like be a friend instead of an uncaring bystander. did you see that one video survellance in NY where a woman was being robbed and the guy took her bag but a homeless man came and took it back and fought off the guy, only to be shot to the ground with noone helping him and so he bled to death.