Is the darting of the eyes back and forth in the behavior of some enraged violent psychopaths due to nystagmus or is it something like somnambulism?

Due to a lack of forebrain activiation, you may be right about somnambulism being the cause of this behaviour; except they actually look as if they're awake while they're doing it. In actual fact, they are in what Dr. Hare describes as a "constant sonmanbulism". Nystagmus' are usually purely physiological problems only.