Because otherwise they have to face up to the fact that they're worthless, unwanted and have nothing to live for.
So they take it out on people who do.
I need to rant.
Okay so I'm walking home from Hassocks Station last night. When I see these two guys harassing a woman, calling her names, spitting on her, trying to hit her etc.
They leave her alone for a bit, she walks down the road looking shocked angry and upset. I stop and think about...