Why do police always want to shoot to kill?

Wildwood - Why do police always want to shoot to kill? I understand if the person is holding a gun but they have choices if the person is far enough away with a knife to shoot their leg or have other officers go behind them while another group of officers are facing the victim.Maybe they need to be better trained.My son went to karate classes and was taught how to defend himself and take away a weapon from someone.The police already know that there is going to be a negative outcome when they kill a black man so why are they not thinking before they kill someone?More defense training is necessary.