A 1933 film starring Lionel Atwill, Melvin Douglas, and Fay Wray. From Wikipedia:
When the villagers of Kleinschloss start dying of blood loss, the town fathers suspect a resurgence of vampirism, but police inspector Karl Breettschneider remains skeptical. Scientist Dr. Otto von Niemann, who cares for the victims, visits a patient who was attacked by a bat, Martha Mueller. Martha is visited by a mentally challenged man named Herman Glieb, who claims he likes bats because they are "soft like cat" and "nice".
On the doctor's journey home, he meets Kringen, one of the townsfolk, who claims to have been attacked by the vampire in the form of a bat, but withheld his story from the town in order to not spread fear. Dr. von Niemann encourages Kringen to tell the townsfolk of his story. Kringen becomes suspicious that Glieb may be the vampire due to his obsession with bats. Glieb lives with bats and collects them off the street.