This argument will never be won with words. You cannot argue something you've never faced first hand. No matter which action an officer takes, it could always be the wrong one. Suppose Tamir Rice had been a small adult and the weapon had been real. A moment's hesitation could cost an officer's life.
The case of the 16 year old girl waving a knife around that's in the news now. Should the officer have waited until she had killed someone?
That situation just happened in Texas on a traffic stop ... an officer was ambushed without a chance.
Read up on the case of Kentucky State Trooper Cameron Ponder.
There are no set answers.
They get slammed for use of force. They get killed when they don't use it.
Yes, there are excessively bad cops. German Bosque for one. There is also a story in the news recently about an officer in Washington state who has killed several and been cleared in all cases. Most officers never fire their sidearms off the range.