I think you are jumbling the terms up, though. "Disturbed" doesnt mean "Crazy". Obviously, there is something wrong with them, but that doesnt mean that they are completely out of touch with reality. This is a great article on the subject:
http://www.crimelibrary.com/serial_k...victims_1.html
I gets into some pretty deep thought about the whole thing. The basic thing is, these people arent living in a fairy world where demons chase them and order them to kill. People like that are exceedingly rare. It is thought that Son of Sam came up with the dog telling him to do it as an excuse.
Most Serial killers are organised, and have a very clear understanding of what they are doing, and that it is morally wrong. They also know full well it is illeagle. That is why these people never get off on insanity, they ARE sane. They took steps to cover their tracks, they know society deems it wrong, but they dont care, they just need to fullfill their fantasy and experience the thrill.
Michael Meyers and Jason are not based on any real people, they are entirely fuictional. Mass murderers of that kind are also exceedingly rare. The only person that qualifies that i can think of is Edward Speck. he killed 7 or 8 women in one night. He also wasnt "Crazy", he was angry.
Just because they cant control their urges doesnt excuse them, or make them fruitcakes. They are usually very methodical. a lot of planning goes into their activities. Nuts dont plan, they react, or act out. Sociopaths are unable to relate to other people, or feel empathy. That may make the aberrant, and even psychotic, but they still "exist" in our world. They dont roam around in little worlds of their own construct, only to wake up and find they have killed. They Meant to do it.