Frankenstein Must Be Destroyed (1969)

What it's about

Blackmailing a young couple to assist with his horrific experiments the Baron, desperate for vital medical data, abducts a man from an insane asylum. On route the abductee dies and the Baron and his assistant transplant his brain into a corpse. The creature is tormented by a trapped soul in an alien shell and, after a visit to his wife who violently rejects his monstrous form, the creature wreaks his revenge on the perpetrator of his misery: Baron Frankenstein.