It's about narrative. It's obvious enough that Mike can't have killed Walt in that moment. It would have ruined the show. But it's the perpetual responsibility of the writers, producers, directors and actors to form a cogent narrative. That is, every action means something and every action makes sense.
If we take it as a rule (and we have) that Mike can't kill Walt 9 episodes before the show ends, then they can't write a moment where he has the perfect means, motive and opportunity to do so. The man was wounded, but not so much so that he couldn't have lifted a gun and pulled the trigger. After all, he managed to crawl away from the car.