Werewolves. People by day, bloodthirsty beasts by night.
They usually live in groups of 2-4, in order to keep down competition for hunting grounds.
Some werewolves enjoy their curse, and take pleasure in the murder of people.
Others are horrified by this curse and try their hardest to keep their curse at bay and never harm people.
Depending on these factors, any given day you can find a werewolf ripping villagers into peices, or a werewolf simply trying to live their lives the way they were before the curse.
Werewolves and vampires are natural enemies due to competition for hunting grounds.