Let me be abundantly clear:
Yes, I am aware that any gun fired at any time can cause death. That’s why I’m generally against carrying a firearm. However, the question did not give me the option of not having a gun.
In the situation posted, during which my pulling the trigger is the only way to prevent one person firing on another, assuming that this is someone I want to save (and not, say, a police officer about to fire on a person holding a child hostage or some equally improbable circumstance), I would be willing to fire. However I would not be willing to intentionally take a life for any reason. There are a great many parts of the body in which one can be struck without causing death before medical intervention is possible. My hope would be to hit one of those areas, and then immediately call emergency services. Of course there is no guarantee a bullet wouldn’t travel or spread, but that would not be my goal.
The end-game is to save a life, not to end one.