I absolutely agree with Dwight and Mike, and constantly remind myself of the same: my carrying a self defense weapon is for SELF DEFENSE. It does not allow me to impose what I perceive as justice on a situation which I am more than likely VERY unwise to. Call the appropriate authorities
Having said that, if I see a psycho with a knife/gun shooting/stabbing seemingly at will, you bet it's two in the chest and one in the head, no questions asked. I consider that self defense, as I am then in immediate physical danger, to say nothing of those around me.
You've got a good premise here though- the line between where deadly force is authorized and where it is not is, when it comes to practical situations, perhaps not as well defined as it should be. This is likely due to the vast amount of variables which need to be accounted for. If somebody is already in the act of murder, deadly force is absolutely appropriate, but it's also too late. What could have been done to prevent that, and where is the line? I'm not going to shoot a common crook in a grocery store, but if he starts shooting, I'm firing back.
"Freedom and reason make us men; take these away, what are we then?"
-Sometimes your best defense is a good reminder of your common sense-
Springfield Loaded 1911 - HK USP Compact 9mm - SA XDM 4.5 .45 - Browning Medalist .22 - Ruger 10/22 - Browning Superposed O/U - Remington 1100 - Remington 720 .270