What's the difference between murderous contempt for life and and insanity?
IMHO anyone who takes a life in cold blood is insane for doing such a thing, thus there is no difference between a murderer and an insane person.
A sane person wouldn't even conceive the notion of placing themselves in a situation to be judged as sane or insane would they?
A murderer is what they are, a taker of life in cold blood, that is a fact, the idea of the murderers state of mind is speculation only, which is derived from experts in the field of psychology. Some of which could be considered insane by their own standards but would never be found to be insane for they are the standard makers.
Jees, I'm going to bed before my own post drives my tired mind insane!