The Ape (1940)

Dr. Bernard Adrian (Boris Karloff) is researching a way to help a wheelchair-bound patient, Frances (Maris Wrixon), use her legs again. The only way he can achieve this is by procuring spinal fluid from a human. When an ape escapes from a circus, Dr. Adrian kills it in a confrontation and successfully uses its spinal fluid on Frances -- but he needs more. Using the ape's body as a suit, he kills townspeople in a crazed attempt to complete his plan.