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.
"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."