When Does a Threat Cross the Line?

I was intrigued with the conversation between the SUFA members around the shooting of the 15 year on the Mexican border. Our esteemed antagonist, Black Flag, took many of us to task for our positions. As is always the case for me, I do not fall into the category of someone who has a knee jerk reaction to things. I tend to think them out and try to eliminate my biases. This situation was no different. And listening to BF’s arguments was important, because I like to analyze my positions and see if I have come to an incorrect conclusion. In doing so I found that what I need to do is define, for myself, when a person is justified in using deadly force in self defense. This article is not meant to further debate the Israeli flotilla incident or the border incident. I will touch on those, but I am more interested in the general answer than I am in an answer for a specific case or two. The question becomes: How do we determine when it is justified to use deadly force in self defense?
Taking the Emotion Out and Evaluating Facts