Ok here's a way to pvp enable the opposite faction if you want. Click on the person you want to enable, then type /duel. While you're dueling type /concede or /forfeit right before they hit you and they'll be pvp enabled and you can kill them.

1) Play as an engineer
2) Attack a warrior or rouge friend in a large pvp fight.
3) Use Mind control cap on them and hit beastial wing.
4) Beastial wing's effect will stay on them.
5) It will break mind control on them.
6) Easy killings for your friend.