You might have seen movies were randomly a demon enters a person's body doing all sorts of weird things just like an idiot...
Well of course it is incorrect. Yes a demon can possess a body although he won't be able to without permission from Satan which he gets from God. Possessions are a rare thing (if they happen actually) and demons posses mainly pure souls making them go through tasks and see if they get corrupted.
Possessing a body does not mean changing the behavior to those shitty CGI you see and elevating, but rather change the personality to doing minor bad things of the individual being possessed.
Whom to blame with possessions is God. Why to give permission of possessions to pure souls if they are "pure"? Also doesn't he know everything since he is God?
Those are questions you should do to yourself.
Ave Satan.