I agree with you except that drowning them is mean. Just shoot them first, two in the heart, one in the head. I have always thought that insanity should not be an excuse. There are lots of crazy people out there who don’t commit these sorts of crimes. And as GS pointed out, sociopaths are quite capable of faking mental illness when it suits them.
I guess I'm just a mean man. I have always thought that what is good for the goose, is good for the gander. Why is it a psycho can torture his victims to death over minutes, hours or days; but he can demand a quick and easy death? I see nothing wrong with drowning them; let them feel the fear of the inevitable, the struggle to live, the horror of knowing that you have to breathe sometime - after all, they had no problem introducing horror, fear and pain on their victim.