From a Christian perspective we know that people can get possessed and are not themselves. Modern words like psychopaths, narcissist, egomaniac etc are words to describe what our forefathers called ‘possessed by the devil’ and what the Bible speaks on many occasions.
My favorite one is when Jesus cast the demons away from the possessed man into the pigs who then committed suicide. We have discussed this detail in our weekly Bible studies and we haven’t come up with anything concrete as to why are the ‘rules’ of the game like this. Why didn’t Jesus destroy Legion right there and then? Why did He have to have them cast into pigs who then killed themselves?
We came up with many answers which we can discuss another time if you wish, but the point is that the evil one is going to torment a lot of people for now but his ultimate fate is sealed.