Why does it have to have some kind of metaphysical source? How do you even prove that? History is filled with people who have done terrible things. In my opinion, using the words evil or possessed to describe violence is a way to distance ourselves from the darker side of human nature(I use the term loosely) by externalize responsibility and blame.
I think both can exist together. Can a positive force work through "good" actions? Can a negative force work through "bad" actions?
Even in our society today we cannot accept our "dark passenger" as Dexter would call it. Humankind is capable of some of the most wicked acts imaginable, and still thinking up more. Yet, we are capable of goodness...and sometimes within that goodness, we have to do acts that are deemed "evil" if done by lesser men and women. Killing, maiming, torturing, etc.
Think of the irony that the soldier who kills on the battle field, or the man who kills his daughter's rapist and killer is considered a just and righteous man? Whereas the person who committed said acts, and uses the same form of violence is considered "evil" and a "monster" (and rightly so)?
Humans, so complex are we.
