The issue of whether it was justified is really what matters.
It will be interesting to, eventually, hear Officer Wilson's testimony about drawing and firing his sidearm as he did.
When you feel like your life is threatened firing multiple rounds is understandable for those who've experienced the sensation. I once had a hunting experience when a wild boar turned for a head-on charge. There were two of us, and we suspected a hog was in bush ahead of us based on rustling of folage. When that thing finally emerged he was charging from not more than 30 feet away. It was a bit unnerving to see a 500lb critter charging straight at us, presumably to inflict harm. In that instance a rifle is worthless. My friend had a rifle in hand and I had a semi-automatic sidearm out, just for such an instance. That critter took several rounds before he even slowed. Every shot fired hit that animal, and he kept on coming until he fell dead in his tracks. When it was over I had 2 of 15 rounds left in my weapon. The whole thing couldn't have lasted more than 3-4 seconds.
We weren't on the hunt for ferral pigs. We were deer hunting, but we knew wild hogs were in the area hence our sidearms. So the event was predictable but surprising.
Of course this discussion is about firing on a human being, and accordingly there is a higher threshold value for choosing when to draw and then when to fire. That said, once decision is made based on fear of life, firing multiple rounds is understandable from my perspective. Honestly, I don't know how many rounds remained in Officer Wilson's sidearm, but if he felt like his life was threatened and he had half his rounds left I'd say his training probably held him back, which is a good thing. In my case I'm sure at least 2 of my rounds were fired after that hog hit the dirt. It sounds like Officer Wilson ceased fire the moment Mr. Brown was down. But, again, it will be interesting to hear what they man has to say about the incident.