I think both evil and insanity are a continuum. Evil ranges from the extremely minor (e.g. driving 5 mph over the speed limit) to the extreme (e.g. Hitler and Stalin). BTW, we are all evil, at least a little bit, including me.Are you going to address the issue of "evil is evil" as it relates to seemingly evil deeds done by insane people (which could actually lead to a somewhat interesting discussion)
Likewise insane ranges from extremely minor (fear of heights) to extreme (e.g. severe schizophrenia). And for the record, we are all insane, at least a little bit, including me.
Some people, like the POS shooter in Charleston commit heinous and evil acts. And yes, most of them have some degree of mental illness. But I believe we are all still responsible for our evil actions, even when we have some degree of mental illness. If I were on a jury, and the murderer tried to claim he was not guilty because he was depressed because his father left him as a boy, or his mother weaned him too early, or girlfriend dumped him, I'd be even more inclined to throw the book at him, not less.
The only exception is that very rare case when the defendant's mental illness is so extreme and so severe that he truly can't tell the difference between right and wrong. They think, for example, that they are shooting laser beams at invading aliens from space. Only a tiny percentage of killers fall into that category. And even in that rare case, the mentally ill person needs to be locked up forever, and just wearing a hospital gown instead of an orange jumpsuit. Society needs to be protected by crazy killers as much as not-so-crazy killers.