One on a message board who takes care of trolls by reporting them, getting large numbers of users to block them, or humiliating them. There are troll killers who rise to the occasion as needed and there are troll killers who simply do it for fun.
That guy just BRUNED the troll, he's such a troll killer.
A good friend who while out with a group of friends will take it for the team and go for the troll of the group of girls. If the troll is not having fun she will make sure her friends no and they will all leave. So you always have a troll killer in your group to take the ugly ones.
Chris: Yo, Luke there is your girl for tonight. You better keep her interested.
Luke: Damn, man I didn't want to take the troll.
Chris: That's why we call you the troll killer!