If in the heat of the moment you continue firing, I don’t think there’s anything anyone would do about it.
If you go get another gun, you need to be able to explain why you thought your life was still in danger.
I am trying to extrapolate the circumstances as you have related them. If I’m mischaracterizing the events, let me know.
While serious activity had take place prior to obtaining the new gun, you do have to explain why you were still in fear for your life, if you fire more shots.
Call the cops. Keep your eye on the guy who is down. If he tries to get and once again shows intent to harm you, start shooting again.
I believe that most people would agree, this is the standard you have to maintain.
He ran out of ammo and the other aggressive perp's location was unknown. That's enough.
This criminal episode was clearly NOT over. NO ONE had been disarmed. The police had not arrived to take over. The scene was still unsecured. The other perp was still at large in the vicinity.
Most independent pharmacies are relatively small, with few places to hide or flee to. Considering the aggressive, violent actions of the perps, you had more than rational reason to believe they would continue to be aggressive and violent.
The cops WERE called. But as always, when seconds matter, the police are only minutes away.