Killer Bats (1940)

Dr. Paul Carruthers (Bela Lugosi) is frustrated because he thinks his employers, Mary Heath (Suzanne Kaaren) and Henry Morton (Guy Usher), have cheated him out of the company's profits. He decides to get revenge by altering bats to grow twice their normal size and training them to attack when they smell a perfume of his own making. He mixes the perfume into a lotion, which he offers as a gift to Mary and Henry. When they turn up dead, a newspaper reporter (Dave O'Brien) decides to investigate.