Referring to the OP's example of Halo, you play as a dying breed on the verge of leading a mass genocide on not one, or two, but three, or even more, races purely because of religious ideals. Shit's crazy man.
Hell, I don't think ESRB is strict enough. While I don't want to see their ratings become law and regulate gaming, I don't like kids having access to all these games. I don't mean that from a protective point of view, but from a jealous one. When I was young, I had to work my way up. There was no starting with the most violent and hardcore game out there, I had to hook up my NES and play some Ms. Pac-man. I couldn't jump right into Pitfall, or god-forbid, Metroid. It was a learning process. I don't think children are ready for gaming, because they come to expect so much from it. It becomes part of them when they can just start with some high tier games from childbirth, because they come to enjoy them, and then end up holding games to a higher standard than say, learning their Bill of Rights. They need to start with shitty games, that way they can shut up and learn to enjoy other things in life.
Also, there's no law saying you can't play M rated games. Just have your mom buy it for you or come in with you. I remember when I was young, picking up Doom and Quake, I just had my dad come in with me. Hell, my little sister had me shrug my shoulders and smirk at the cashier when she bought Oblivion, that was good enough for her.