This is not an exact match, but Vincent Price's character in 1963's THE HAUNTED PALACE is possessed by his evil ancestor. We think he regains control, but the ending hints very heavily that the dead spirit is once again in possession.

I think the best "he's not dead" bit was not in a horror film but in the wonderful neo-noir RED ROCK WEST. Dennis Hopper's paid assassin is "killed," but the film plays fair, showing a flicker of his eyelid (or something like that) to foreshadow his resurrection a moment or two later. Consequently, the scene feels more believable, not like a movie cliche.