I don't know, Chris. I found it refreshing that Marty refused to play the utilitarian situational ethics game. Shooting your innocent friend in the head, even to save the world, is not okay. It's just not. You're not actively causing the destruction of the world by. You are actively murdering someone. Marty opts out of the game because the rules suck.
I don't think that Marty necessarily wants the world to end with him. I think that he doesn't want it to continue under the conditions that have been outlined for him. There's not much point in saving a people that gets by by ritually murdering its own. There's not much point in saving humanity when humanity has become inhuman.
Anyhow, it was time for mankind to move aside and Make Way For Goslings.