I do disagree with the idea that only mentally ill people kill. Fighting and even killing is a natural instict in the wild to protect territory, claim breeding rights, ensure strength of the group and safety from outsiders. Like it or not, people are animals. "Civilized" behavior has to be taught. Little kids are mean as hell until they're taught to behave by their parents - and they're not mentally ill. They're just doing what nature has programmed them to do until they learn the rules of society.
So once again, it goes back to my opinion that the moral decline in our society falls squarely on the shoulders of parents. They are not properly teaching children how to behave and react as civilized human beings, so our man made morals are being lost to animal instincts.