Did he assault the officer?
Yes.
Did he charge the officer?
Yes.
Is he dead as a result of those actions?
Yes.
Case closed.
Next!
Neither of these assertions has yet been proven. There are other theories that explain the actual known facts, though not perhaps as well.
For instance, if the cop has a broken head bone, it is possible Brown gave that to him while legitimately defending himself against an unprovoked attack by the cop. Admittedly, not very likely.
We also have, to my knowledge, no indisputable evidence that he was charging the cop when killed. Though I suspect that's exactly what happened, it hasn't been proven yet.