Police use of force for the most part mirrors civilian use of force.

[X_Digger]

There are a few circumstances that generally justify an officer's use of force that don't apply to civilians (and that varies by jurisdiction) but those are rare.

There is no requirement to meet force with the same kind of force. Once force that would be reasonably expected to cause death or grave injury (given the totality of the circumstances) is used by an attacker, lethal force is justified in return. Defense of another, especially in stopping the commission of a forcible felony (kidnapping, rape, arson, etc) are also typical justifications for deadly force that apply to all.