The Ghost of Frankenstein (1942) on Netflix

The undead Monster and the wily Ygor have survived another mob of angry villagers. Needing help to restore the Monster's strength, Ygor brings his hulking friend to Ludwig Frankenstein, the original doctor's son. Ludwig has built a life and a reputation for himself in another town, far from the stigma and shame that his father and brother brought on the family. Ygor blackmails him. The good Ludwig realizes that the Monster is evil and must be destroyed, but he struggles with the idea of taking a life, even an undead one. He finds a solution: replace the Monster's criminal brain with the brain of his recent victim, the noble Doctor Kettering. But sly Ygor is a step ahead: he manipulates Ludwig's colleague, the jealous Doctor Bohmer, into substituting Ygor's brain for Kettering's. After the operation, the Monster--with his physical strength now guided by Ygor's wicked brain--is more terrible than ever. When will those stuffy scientists finally learn? Luckily, there is a mob of angry villagers outside Frankenstein's house...