I agree that violent tendencies should not be the focus when talking of pyschopathy. In answer to the question of why; I'd have to say it's because violence has the potential to have the most detrimental effect on individuals and society as a whole.
IMO the salient issue with pyschopaths is the lack of empathy, or often acknowledgement that others have validity or even exist. A significant lack of emotional involvement in their own lives is probably the reason for this lack of empathy. It's hard for a person to care about the plight of others when they themselves don't particularly care about their own existence, or the ending thereof. Psychopaths have little or no emotional response, and have a tendency to see others as being puzzlingly emotional. The author is making an attempt to pretend like he cares in order to please those around him that do care.
I imagine that a pyschopath might engage in violence in order to try to feel something, anything, even horror is better than . . . nothing.