Pro-gun proponents argue for more people to be armed so they can repel attempted mass murders before they eventuate.

I was playing this out in my head to see how it would work in reality.

Suppose a gunman opens fire on someone at work and kills them. The only other person in the area takes out his gun and kills the shooter. The noise is heard by others who rush into the room to find the second shooter standing there with a gun in his hand with two dead bodies at his feet.

How do people know which of the three was the culprit? They don't know which is the culprit and aren't aware that one of the killings was a 'rescue-shoot', so they assume the live person is the culprit and shoot him too, since he has a smoking gun in his hand.

Do the pro-gun proponents have any suggestions on how to solve this one?