No, they are supposed to arrest every criminal that tries to kill them. Sometimes they have no choice but to shoot (and maybe kill) someone to protect themselves or others, but that is not the goal.

The inverse is true though. They are not supposed to kill any person that doesn't try to kill them.

No they aren't. They are not mercenaries. They are supposed to arrest every suspected criminal and prevent those suspects from committing acts of violence. They are allowed to use violence to prevent loss of life, but they are not supposed to kill anyone.

The point is, if every one of those deaths were someone who presented a credible threat and were actually trying to kill an officer, why do so little of them succeed? If anything, they have the element of surprise on their side. Police are often only given little ammunition and training time every year, literally just a couple boxes of ammunition. Anyone just casually going to a gun range every couple of months for fun is going to have more experience than most law enforcement officers.

If they were only killing every criminal that actually attempted to kill them and had the means to, there's no chance the defenders in that situation are coming out successful 20 to 1.

It would be better to capture those criminals and dismantle their networks.