Back in the day, the legal excuse of choice used to be “temporary insanity,” which always seems to me to be accurate, as anyone committing murder is by definition temporarily insane.
Now we have the ultimate airtight excuse for committing murder: “ I am the victim here as THE COOKIE MADE ME DO IT.”
I used to believe this. Now I see quite clearly that there are people who know right from wrong but simply don't care. They kill other people as you or I might kill a fly and suffer no remorse or regret whatever. They might regret being caught and punished.
I have decided that there is such a thing as "evil". Assuming that an evil person is insane is to suggest that something might cure it. I believe that some people's brains are simply mis-wired. They will always be with us.