Well, see for a police officer to engage in deadly force use against a woud be criminal, there has to be 3 elements met. It's called O.C.I. I'll explain:
Oportunity= meaning that the person is physically there.
Capability= Having a deadly weapon gun, knife, rifle, shotgun, etc.
Intent= Means that the perpetrator points the weapon at the officer or physically charges at the officer with a deadly weapon.
Now if one of this elements is not met, the officer must use other non deadly means to get the person under control, pepper spray, baton, stun gun, etc.
If he doesn't he is pretty much fucked by the law.