If Officer Wilson's version is factual (MB punched him in the face, tried to wrestle his firearm away from him, a shot went off in the car), from that moment on MB was a highly dangerous suspect, even if MB retreated momentarily. If MB then (for whatever reason) turned around and began to approach the officer (even if he only took a few steps toward Officer Wilson), he was still a highly dangerous suspect due to his previous actions seconds before.
MB had already punched the officer and tried to seize the officer's weapon. If this is true (I believe it is), then, as far as I'm concerned, there was an extremely strong probability that if MB got close enough to Officer Wilson again he would have tried again to gain possession of his firearm, unless immediate action was taken to stop him.
It's been reported that MB was shot 6 times. It's also been reported upthread that the type of gun Officer Wilson was issued holds a total of 13 rounds (12 in the clip, 1 in the chamber). Although we don't know how many rounds DW fired, it's more than likely there were still bullets left in the clip.
I don't have difficulty envisioning that DW fired only enough rounds to stop the threat, no more and no less.
I've heard so many comments in the various media from random community members who've asked "Why did he shoot him so many times? Why didn't he just shoot him once?"
Apparently, one shot wasn't enough to stop the threat, because that 1st shot struck MB in the right arm, as did the 2nd, 3rd, and 4th shots, according to Baden's autopsy diagram.
Based on what's been reported thus far, I don't have to stretch my imagination to believe that if Officer Wilson would not have shot and killed MB, it's more than likely that Officer Wilson would have been shot and killed with his own gun and there would be one more name to add to the list of law enforcement fatalities on the Officer Down Memorial Page.
