Maybe the Manhunt games. People often accuse games of being 'murder simulators' but in this case it is actually true. I think it is the fact that the entire game is built around this concept that makes it so controversial. Then again I guess you could call it art in a twisted sort of way.
On the subject of Gears of War, I didn't find the Chainsawing and exploding heads to be to bad. It wasn't like you were chainsawing every single enemy in the game. However, what for me went 'too far' in terms of blood and such was a certain level in Gears 2. People wwho have played it know what I mean. It just seemed very excessive to me, but that's my own opinion.