What is the difference in evil and mental illness?

Mental illness is something that can occur in an infant (from a difficulty during birth from the mother) and can show as the child grows and develops. Evil is people like the late James Gacy who his evil tendencies under a cloud of friendliness but had this "need" to kill boys who accepted his offers of jobs and ended up being bound, strangled, and killed.

Mental illness can also occur during a person's lifetime from some traumatic experience. No one knows for sure until an autopsy is done.