It's mental illness folks. We don't accept it because it's abhorrent behavior but these people need mental health help. Getting them to seek help is a nearly impossible task though.
I agree; I'd imagine the act of violent/abusive trolling is a clear symptom of one of a number of personality disorders.
Ultimately, everyone has their foibles. It's what makes humans human. Only when someone become Internet Famous, though, do we start diagnosing them with illnesses. Why is that? Why can't we say, "wow, this guy has chosen a pretty worthless hobby. too bad he can't use his time to build model trains or something"?