how is it not his fault that he behaves like a piece of stuff
Because he wants attention, and people keep giving it to him negatively, and so he responds in the same way. The comments he makes are only seen as worse than the people who are insulting him because he has status.
And when I say he wants attention, I don't mean he's making a conscious choice. If I could choose to stop being a richardhead half the time I'm on these forums and other ones I would, but I can't. It's the way his personality is and it's not easy to break out of. It's not an ultimate defence clause, but hopefully you're starting to see why he stuffposts. I bet at the end of the day he doesn't really believe in or care about most of the stuff he says on his Twitter.
how is it not his fault that he is incapable of dealing with his "stress" in a healthy manner
Alcoholism is a serious, significant problem all over the world. It's the same with many other drugs, as well as Self Delete and self harm. People have different ways to resolve their problems, and without a voice to say yes or no, you can't really know better.
That's how he deals with it. Hopefully he gets help eventually, but it's not exactly something he can just break out of.