Any time an assault occurs and an officer is involved there is a weapon involved. An officer cannot lose, understand that, because if they do their assumption is that you aim to use that weapon against them. If the tide in a physical confrontation reaches a point where they feel they are losing, they will very likely draw their weapon and shoot the aggressor.

Michael Brown had a distinct size advantage, and he used that minutes before when her robbed the store (robbery = theft + force), and again in his altercation with the officer that shot him.

The officer isn't acting as executioner. The officer is defending his or her own life. It is that simple.