Well, I agree a guy's got to have a couple screws loose to keep coming toward three cops who have their guns drawn, refusing their repeated orders to take his hands out of his pockets, when he knows they're aware he's got a knife. But although cops try to help people who are in a bad way when they can, they are not social workers. And although I'm sure they would much rather be able to get suspects to comply with their orders and have things end peacefully, there is a good reason they're equipped with sidearms. Some people can't be reasoned with.
The cops I've met are mostly reasonable people, and I'm sure about the last thing most of them want is to know they once had to kill a guy who was probably not all there. Anyone who thinks the average cop gets a kick out of killing a guy he had nothing against should listen to Sugar Ray Robinson some time, talking about how it felt to throw a punch that killed an opponent in the ring. It shook him enough that for a while after that he couldn't bring himself to hit anyone very hard.
They can also be lethal.
Video: Crazed man dies after resisting TASER, attacking cops
"Political speech and writing are largely the defense of the indefensible. . . . Thus political language has to consist largely of euphemism, question-begging and sheer cloudy vagueness."
~Orwell, Politics and the English Language