Who started the fight?
Don't know ... don't know if we will ever know ... but that is up to a jury to decide, not racist assholes like Al Sharpton.
Excessive force?
Don't know ... again, that is for a jury to decide ... but if a man is wearing his knuckles out on your face, has broken your nose, and has you down and is pounding the back of your head into the ground, would you consider it "excessive" to shoot him?