You're approaching this as if any risk at all is unacceptable. I disagree, I think the element of risk is an essential part of the job description.
I expect cops to put themselves in danger, at least a little bit, to try to make the system as fair as possible. That is to say, the purpose of the police is simply to get the suspect secured and in front of a judge, where the actual factfinding is to take place.
Cops are supposed to get a little dirty while executing their duties. They have to deal with domestic violence situations, crazy hobos, tackling drunk guys at bars, etc. Like if the other guy has a gun, sure, go for it, light 'em up. But just a knife? That's going to be a case by case basis, and I've seen too much bullshit from cops to go easy on them.
They should be compensated appropriately for the risk, of course. But a cop who just knee-jerk shoots a guy with a knife in every situation? That's like a firefighter who won't run into a burning building.