If Brown was attacking Wilson then Wilson has to protect himself. If Brown gets his gun then he is a threat to the officer and the public. Police are trained to go for their gun and why wouldn’t his hand already be there if Brown was going for it? Police do not keep rubber bullets loaded in their gun. Police, and any self-defense course, will teach you to shoot until the threat is down, a 6’4” nearly 300 pound man can take a few rounds to go down from some handguns. Also, you have to think of the reaction time it takes in a stressful situation for your brain to tell your finger to stop pulling the trigger.