It's only as bad as you make it to be. You can just go through the whole level without shooting your gun, not kill anyone and just watch the slaughter. It was made for the player to hate the terrorists on a personal level, however i also do think that it was created to get more media attention then god.
Both of them are true, we all know it.
Plus you can, you know, skip the damned mission and the game warns you twice that its disturbing content.
EDIT: You do have to kill the police once they arive, but you DONT have to kill any civi's at all, you can just walk through that part of the mission.