Satan ?

"'There it lies, I think, Damien . . . possession; not in wars, as some tend to believe; not so much; and very seldom in extraordinary interventions such as here . . . this girl . . . this poor child. No, I see it most often in the little things, Damien; in the senseless, petty spites; the misunderstandings; the cruel and cutting word that leaps unbidden to the tongue between friends. Between lovers. Enough of these,' Merrin whispered, 'and we have no need of Satan to manage our wars; these we manage for ourselves . . . for ourselves . . .'"

An excerpt from The Exorcist by William Peter Blatty