But, had you been carrying a gun under those circumstances, you’d be covered under the Just War doctrine of the Church, would you not? You are allowed to use escalating force for the protection of the innocent and unprotected. If it was me, I would attempt to mame, rather than kill. I have qualified with several guns while in the military, and I am confident in my abilities. Therefore, it would be merciful for me to shoot the perpetrator in a non-fatal region, but, if my opportunities have been exhausted, I’d unfortuantely have to kill to protect the innocent.
Now, the next question would be: would I feel guilty? I don’t know. Maybe. I would feel horrible that I had shot and killed a person, but I would know that I prevented the murder of possible many more innocent individuals.
I would still go to confession for it, to be sure.