A vicious animal attacks you on a moonlit night. Friends drive the beast off, but you discover that you've been cursed with lycanthropy, to become a similar beast under the full moon. (work with your DM to determine details)
Martial training: In adolescence you trained with the weapons of your people and learnt how to use them proficiently (work with your DM to determine details)
You become involved with criminal elements of society, taking to a life of crime and earning a notorious reputation and bounty. (work with your DM to determine details)