During my first job as a psychologist at a psychiatric hospital, I told my psychoanalyst/supervisor my adolescent patient's statement. "That's psychotic," the doctor replied. Though able to define "psychotic," until that moment I hadn't grasped the power of this condition.
Similarly, when mass murder horrors become public, the perpetrators are often viewed with surprise for these men had seemed so normal. They lacked the twisted features of horror film characters and spoke coherently though of bogus beliefs. Columnists asked the usual question of "why," and provide their usual answer that "no one knows." But this is not true!
While predicting violence cannot be certain, it correlates highly with several factors: failure in life; substance abuse; the ego capacities governing thinking and behavior being inadequately developed; and having a fragmented "sense of self," sense of who they are. The killer's frequent decision, to suicide in "glory," is considered preferable to their continued painful existence.
Though their act was horrendous, these individuals are not often considered "insane." The legal definition of insanity is determined by state statute, most usually whether a person can distinguish "right" from "wrong," and rarely succeeds as a defense. But to describe them as sane does not imply that they possessed "normal" control over their behavior though, except for those with extreme limitations, this should not influence their punishment. There is evil in the world and some succumb to its temptation. Yet even for the rest of us, the unconscious is very powerful and one must respect its power.