Looking for Satan (and finding ourselves)

Were Hitler and Stalin possessed by Satan?  The Vatican's exorcist -- is that really an official position? -- apparently believes so.  Now I'll be the first to count The Screwtape Letters as a wonderful reminder of the spiritual dimension to good and evil, but pronouncing with any confidence when human evil is the product of satanic power threatens to marginalize the human choice from which evil results.  "The Devil made him do it," in a way, serves to disconnect us from the mess we create, minimizing our moral agency.

Rob

UPDATE: A reader reminds me of 1 John 5:19 ("[T]he whole world is under the control of the evil one.") and asks

Isn't all evil the product of satanic power? I don't think pointing that out minimizes our human agency, as long as we're compatibilists about human freedom (as we should be).  As I see it, saying that God makes people do good things doesn't take away genuine praise of those people, and saying that the Devil makes people do bad things doesn't take away genuine condemnation of those people.

I agree generally, but if Satan's hand is everywhere, I wonder whether it makes sense to distinguish Hitler and Stalin from everyone else.  And I still question what blaming Satan does to the moral culpability of the possessed individuals.  if the really, really evil folks are possessed by Satan to a greater extent than the rest of us somewhat evil folks, are the really evil ones culpable because they’ve allowed themselves to fall into Satan’s possession, or are they culpable because they still exercise control over their own actions?  If the latter, what is Satan’s role – a matter of influence, rather than possession?  I don’t at all deny the spiritual world and its relevance, I’m just much more reluctant than the exorcist is to identify particular instances of its manifestation.