Very very few murderers are insane.
And yes, some of them do just get up one day and decided to kill someone........even their own mothers or fathers.
It happens all the time unfortunately.
IMO
Maybe it's just me - I think they're all insane! I look at the "talk back" mom who killed her 2 young children recently because they talked backed to her. She premediated this crime by buying the gun, then had to wait 3 days to get it. Then wrote the rambling letters. Those are the killers I'm talking about, and I think (IMHO) we see more of those than the ones who kill on a whim (ie, the Arizona killer). He's a complete nutter too if ya ask me (again MOO).
I know I could probably come up with a few who kill on a whim, but I just can't think of any - especially children vs. parents.
Best,
Mel