Yeah I agree that it doesn't actually matter whether they are literally demons or possessed by demons, the result is the same. There is a common element in all studies of evil that it's a supernatural force that's of one source but it manifests as separate entities, as if it's capable of jumping from one person to another, kind of like in the Matrix. When a person becomes evil they become a tentacle of evil. It doesn't make sense to assume that every evil person is an individual whose evil resides only in themselves. It's the opposite: they are united with all other evil people through this process of allowing themselves to be controlled by this force that's driving their behavior from behind a veil. This concept has been explored in many books and films. The TV show Millennium that ran from 1996-1999 explores this concept in some detail.