I still say this is a case of excessive force.
I think it all depends on whether or not one version of events we heard is substantially true or not. That version is that Brown attacked the initially then moved away. Then when the officer moved away, the version goes, Brown moved towards him in a threatening way and acted like he wasn't going to stop.
If that happened it wasn't excessive force. And I say that even though I'm a person who thinks the system allows police to use excessive force all the time. I think they're allowed to push citizens around and get rough way too much.
But if a 6'4", 300 pound guy who already attacked and significantly injured you is charging you I'm not saying you used excessive force if you use a gun that you have to stop him.