The Werewolf Syndrome: Compulsive Bestial Slaughterers
There have been many explanations for the werewolf delusion, from the disease porphyria in which people develop photosensitivity, to rabies, to consuming hallucinogens via rancid grains. The "cures" involved everything from purging to rubbing opium in the nostrils. Yet none of those cures would affect someone suffering from mood or psychotic disorders. Nor change the appetites of a sexual predator given to violence. It's possible that some "werewolves" were simply sadistic killers who had found a culturally-formed image through which to act out their fantasies.
Dr. Stephen Giannangelo addresses serial killers who derive a joy from their killing sprees in The Psychopathology of Serial Murder. He says that they experience a "pervasive lost sense of self and intimacy, an inadequacy of identity, a feeling of no control." These things then manifest in an ultimate act of control — murder. Such killers develop deviant sexual motivations that become consuming fantasies that issue in an initial murder. When they find reward in that, they continue to look for other opportunities, refining their approach and acting out further deviance. The form it takes is influenced by whatever image or object has a sexual hot button in the fantasy. Bestial paraphilias that encourage savage attacks are obviously potentially dangerous.
Some psychologists have found the cases of lycanthropy to be an interesting study. Far from being nonexistent, there are still people who believe they have changed into beasts.