The usual way. He's now mind controlled by Kilrogg and we kill him and he says "Thank you, heroes, I'm free blah blah blah". Like we haven't seen this shit a million times before.
All keepers in Ulduar, Keristrasza, Saurfang the Younger, Anub'Arak and dozens more that I can't think of right now. It's along the "corrupted/insane" approach that Blizzard has with all npc enemies.