All depends on what I think the witnesses are likely to say afterwards.
Will they say "Thank God that man was here to save the day!" Or do I think they'll say; "That man shot that boy for NO REASON!"
Problem is that many of these situations happen in seconds...not minutes. There often times will be people standing around outside your view, many with cell phones who are very practiced with taking pictures/videos as has been seen many times. Also in this modern world are surveillance cameras that seem to be everywhere in some areas... These can hang you, help you or even confuse the issue.
I personally like and encourage my students to adopt Flop's philosophy of:
"I will defend, myself, my family, my friends, my congregation, the elderly, a fallen police officer, my brothers and sisters in the military and small children. Any able bodied adult, male or female, is SOL. I will not risk my freedom, or wealth to defend those unwilling, or too stupid to defend themselves."
..my personal problem is that having been a warrior all my life my code of honor will not allow me to sit by and watch others be harmed even if they didn't have the brains to protect themselves...as Lobo said...
"You are correct, the use of reasonably necessary force is generally permissible in defense of oneself or another person. In my opinion, there is no line to be drawn. As human beings we share a responsibility to one another, so any time that another person is in imminent danger of serious bodily injury or death and it is within our abilities to render assistance I believe it to be a duty."
Just the Honorable thing to do...Bob