A difficult conundrum this. Prokofiev wrote some pretty evil music -his Suggestions Diabolique are worth exploring -
Something atonal might also be worth exploring -like Webern -because in this music there is no tonal axis -there is no balance - -but maybe this doesn't quite underline murder musically -except perhaps for the insanity part -
Can I alos suggest something a little more off the wall -you might run into copyright issues though -how about a transcription of a heavy metal rock piece? Check out Vikaygoeswild on YouTube!