This works only if you have hysteria. What you want to do is duel someone in wintergrasp, and beat them so they are at 1 health, then pop hysteria on them after the duel is over and they will die from the damage over time effect. This kill will count towards the quest even though the member was of your own faction. I'm sure that this would work for any similar abilities that drain life, but only if you are the one casting it, meaning hellfiring to death as a lock probably wont work.