I know this because I'm one of them. (I'm working on changing things, though.)
Usually, these people aren't exactly aware of what they're doing. Because they'll tell you things like, "I'm not an angry person. Do you think I go seeking out the things that make me angry? Do you think I make things about me, when they're really not about me at all? I'm not an angry person. It's just that the world gives me so many reasons to be angry. Have you seen it? It's a mess!
(They're really clever about it, you see. Everything they say sounds plausible. Add to it the fact that there are plenty of legitimate reasons to get angry, and you've got all kinds of excuses for self-righteous anger.)
Angry people often feel like victims of something. ("Don't you see, it's not that I'm an angry person. I don't seek these things out. They MAKE me angry!")
The problem with anger isn't that it's not justified. It's often extremely justified. The problem isn't that it's useless--although most of the time, it *is* useless.
The problem is that it's self-destructive. You know the old saying about how anger is like drinking poison and hoping the other person dies. There's a lot of truth in that. There's a lot of self-inflicted pain in anger.