My one resort would be "stop drop the gun" count to five or if he fires or turns toward me. Bang bang bang until they would be neutralized threat over.
For me every situation is different. There is a time for everything. I will live in the moment and trust God to put in my heart what is to be done. I will not hesitate under any circumstance. Any man who understands emergencies knows that hesitation means death. There is no room for hesitation or panic. It's my faith in the Lord that gives me peace and a cool head. With that being said (just to put myself out there for the record) There are situations I can imagine that could easily result in a human losing their life. Make no mistake about it, death comes easy. You could push someone over and they can hit their head on concrete and die. In that instance we will just have to assume that God wanted it that way. All I know is that there is zero chance that I can see a murder taking place and do nothing.. Not only is doing nothing not an option, but losing is not an option. If I am someones last line of defense and I perish I helped them not a bit. I stand by these words
"I feel 100% better about standing in front of God explaining why I killed that man to save that child than I would feel about explaining why I let that child get murdered."
In fact it is exactly that issue that leads to my confidence in this situation. Under no circumstance will I stand before God with that second situation. I absolutely refuse- because I know better. God wouldn't put me there just to watch. If God didn't want that man to die (if that was to be the result) then God could have put it on my heart to go somewhere else, could have put it on that mans heart not to do it- could have snapped my leg when I was walking, hurt that mans back when he rose from his chair, put him in a car wreck on his way or gave me an emergency phone call from a family member. The reality is that everything happened just exactly as it did to land me right in front of a man trying to kill a child and I have but one question to answer "should I do something?" answer "Without a doubt."