Stalkers must be mentally defective to do that to people. How off balance do they have to be and why do some who stalk murder their targets?

We remember John Lennon but do you remember Rebecca Schaeffer? She was in a TV show starring Pam Dawber and played the part of the younger sister. One day she opened her door and was shot to death by her stalker.

What moves a mentally defective misfit to engage in violence?