The devil made me do it....

Every now and then, a story mentioning exorcism, usually one gone bad, surfaces in the mainstream press. Take a look at this search I pulled up search on DisciplesWorld's web site for just a few recent ones. Evil, the devil and the demonic have always been a theological problem...in my mind, THE theological problem. About a week ago, I started reading Michael Cuneo's book, American Exorcism: Expelling Demons in the Land of Plenty . Cuneo, a professor of sociology and anthropology at Fordham U., spent months researching actual exorcisms happening in America. Many are not Catholic - they are Protestant "deliverance" ministries, some of a rather freelance sort. Cuneo's book is fascinating and scary.
A few days ago, I saw where the BTK killer has attributed his actions to a demon that entered him at some point. Now, there was a time when I would have said b-s as fast as you can blink, but now I'm not so sure. The demonic is as good an explanation as anything else for why someone could become a serial killer.
Then today I came across a story on ReligionLink - a resource site for reporters - about a Columbia College psychiatrist who is actually coming up with a 22-level heirarchy of evil (link at bottom of post).
What do others think about evil? Is it a tangible force? Is there a cause behind it (the devil, or God?) Are there demons? Is it like that line in the movie, The Usual Suspects - "The greatest trick the devil ever pulled was convincing the world he didn't exist."
ReligionLink - A hierarchy of heinousness: new views on evil