Stan was previously a good-natured but pretty slow bus attendant who claimed he would be the next Minister of Magic when he was trying to chat up some Veelas. That said, he seemed very unlikely as Death Eater material, even if he did decide to join. On the other hand, Harry and his friends believed that Cornelius Fudge was under the Imperius Curse which was why he was stubbornly ignoring signs of Voldemort's return (Dumbledore doesn't think this is true though and Fudge later does accept what's going on, so yeah...) There's also debate as to whether or not Umbridge was possessed by the horcrux locket when she wore it. While Harry figured that it would be impossible to tell since she was already so foul, Rowling stated that the locket saw her as a kindred spirit and aided her rather then hindered her.
This and the note about Stan Shunpike really contributed nothing. We never learned about whether or not Stan was brainwashed or not.
Cut this from the Harry Potter example:
This and the note about Stan Shunpike really contributed nothing. We never learned about whether or not Stan was brainwashed or not.