Why is my friend acting this way?

One of my guy friends (I'll call him G) is very pessimistic, to say the least. He acts all buddy-buddy with his friends, then talks about how much he hates them as soon as their backs are turned. G says he can't trust anyone, that solitude is the only way to go and that he feels that way because of what happened to him in his childhood. He won't tell me what, but all I know is that his brother drinks a lot.

He has the "F*ck this, f*ck that, f*ck you" attitude, saying he can't wait for the world to end so that all the sh*theads in the world can die and he can laugh at them. G claims he's not suicidal but just waiting for the world to end, and how there isn't a person in the world worth his time because they're all horrible - yet he always preaches to his friends about this, thinking it won't bother them that he's basically talking them down. In fact, when we text, that's ALL he talks about. It's why I stopped talking to him for a long time. G, for some reason, tends to hate a certain type of people who act just like him, but he doesn't seem to notice it. I even when I pointed it out once but he didn't know what I was talking about.

Basically, why would he act this way? Is it an act or is he really that angry all around? I've tried to help him many times, but now I'm thinking of just letting him continue his negative thoughts so he can learn his own lesson over time.