I know this may not sound very tactically sound from the cop's POV (or safe?), but what if the officer had fired a few rounds into the air between bursts of shots fired by the perp to distract him and maybe stop him from shooting innocent people for a moment or two? Yes, it alerts the bad guy that someone else with a gun is coming, but he might panic and just go ahead and off himself if he thinks police are near and it may have at least saved a couple innocents from being shot. If nothing else it disrupts his OODA loop momentarily and maybe gets him off his game. Just trying to think outside the box...
Sorry, but no. You have to account for every round as a cop.
He did exact what he was supposed to do.