I think it says more about the ESRB changing than Halo. I mean, the original game I don't think had much that made it an M—the Halo games have never had that much language and the blood is more cartoony than graphic; people aren't dismembered and strawberry jam isn't chucked onto your screen. Bungie ascribed the original rating to the Flood, but even in Halo 2 and 3 they were never that gruesome to me. The most graphic scene in a Halo game was probably the Composer in Halo 4. I can see it being fairly easy for a Halo game to move towards a teen rating—especially as a larger number of your opponents aren't biological at all but explodey robots.