I think what we can say for certain is that there is a portion of the community with anger issues. Some of them are so severe that it leads me to question their mental health. I don't know what percent of the community suffers from mental health issues but I think it is safe to say at least some do. The question here is to what degree does set of ragers intersect with the set of people with mental health issues.
I'm skeptical of the idea that lack of authority means people will act poorly. See John Locke's State of Nature for an interesting read on this subject. People tend to behave because they recognize that there are consequences at all levels of human interaction. In league of legends the the consequence could be banning, but the immediate consequence could be losing. If you rage at people in your game and cause them to lose the game then you're effectively losing on purpose. Yet, you're mad that you're losing. Does that not sound like complete cognitive dissonance? This is such simple reasoning that it does make me question the mental capacity of these players.