Wow. They are what they choose to be just as everyone else. There are humans that are evil and humans that are good. There are weres that are evil and weres that are good.
Humans are evil. Any werewolf who is "Evil", is so because of the human part in them, and the wolf part doesn't know how to cope like a human does. That is our explanation of why a fictional werewolf would be evil (from over at a private group I am in.)