Personally, I think with Michael we're looking at a hostage situation. He'll damn well do anything to get Walt back. Killing a woman he barely knows, and really doesn't like anyway, is just another stepping stone to getting Walt. And, I really think killing her was just something he had to do in order to free Henry Gale. If no one had been around, he wouldn't have had to shoot anyone. Either way, I think his "mission" was to come back and free Gale, if he wants to see Walt again...
