Yes, there is absolutely no rule against playing an evil character. But.

Be sure you know what you're doing. Remember that the point of this entire thing is to defend Edgekeep against the forces of evil, and that the huge majority of those here are actually interested in doing that. If you decide to be evil, consider whose team you're putting yourself on. Don't be surprised when the reaction you receive from the other characters on the board is somewhat less than friendly. Unless having nearly everyone on the board discussing who gets to kill you is your idea of fun, I would suggest that you either try playing a neutral character (they don't get offed nearly as quickly), or at least be very discreet with your evil. If you don't believe me, ask anyone in town about a chap called Berujomii...