Society is awful and rejects people for being autistic, bipolar, ugly, etc... If you are treated badly, it's easier to be consumed by bitterness and become a negative person yourself. And 4chan attracts any rejects. In addition to that, if you are rude and openly a bad person, not in a deceptive sort of way, not in a way that society doesn't care about, then you are also rejected.
Of course, the anonymity makes this worse too.