Mmmmm - not sure what you are trying to argue here.
It would seem that you are saying that the police should not have done anything until after the dude did something terrible.
Following your reasoning, the police are fukked either way - they are "wrong" if they do something to prevent something bad from happening, and are also wrong if they don't do anything to prevent something bad from happening.
C'mon, it's not that difficult to understand, you don't have to wonder too much.
Let me write it out again.
He got shot and killed, the cops had no choice, they did their job, it was not their fault.
What would have been an ideal outcome, is to detain him, and provide him with assistance - I mean the guy has a family, a wife etc. It doesn't appear that he was a seasoned criminal. Surely underneath all the rage was someone that could have been helped. But unfortunately due to his actions he was taken out.