There are no demons, the mind is just extremely mysterious and Id go as far to say that we know less than 10% of what there is to know about the human brain.
Something like this could be caused by a number of factors though... schizophrenia, neurological disorders, extreme trauma... there are too many factors left out to say that it was caused by a possession of some kind. The exorcism failed didn't it? That should be a clue that possession is a load of crap.
Hell even if there WERE demons, I doubt they'd be capable of the same level of evil that actual humans such as Hitler unleashed upon the world.