Or what about using it as a one off ability and allowing for the victory point? Once per game, the Backstabber can kill an adjacent friendly character and receive a victory point. To further help soften the strength, it could require the use of a non zero combat card. It isn't overly powerful since you'd have one less character to escape.
Thinking about the Backstabber some more.
As a House Rule, I'd give the Backstabber the ability to eliminate an adjacent wounded friendly character for 1 AP. However, no VP would be awarded to either team as a result of this action.
The Backstabber does this primarily to prevent the opposing team from gaining a VP (by attacking the wounded character). I need to decide whether she can do this just once or multiple times during the game.
No VP is awarded to the Backstabber for this action because really, it would be too easy to get VPs this way and I wouldn't want to encourage players to go around slaughtering their own team.