Anyone who is promoting crap like ISIS or other crap like that should be punished. Whether person is serious about that and he/she actually supports those bastards or this person is an idiot who tho thinks that is funny or cool - that person must be punished...I'd slay the crap out of this clan in game and i would beat up someone who is carrying ISIS flag on the street in real life. If you say that it is their right to promote/support this crap, then it is my right to take their lives for doing that.
Of course in games, especially mmorpg we create clans and characters named Killers or LeagueOfAssassins or something like that and everyone is okay with it. But I don't think that promoting real organisation like ISIS or KKK that is not made up by a player or seen in movies or elsewhere should be O.K