The legal issue will be the bum-rush and whether that was justification to resort to lethal force? I do not know the answer to the question, but I think it is going to be problematic. Did the officer have a night stick? Did he have mace? Any other nonlethal weapons available? Officers are trained on continuum of force before they go to lethal force.
If Brown had been a normal sized guy then the officer might have had options.
However: he was huge and powerful. It would have been like using a nightstick on a charging bear.
Did the officer have a night stick? Did he have mace? Any other nonlethal weapons available?
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Would be a bit difficult to use such, after a 6’4”, 290# guy shoves you back into the cruiser, is pounding you in the face and trying to grab your gun.