This is a very important issue, and it is disappointing to see that people do not wish to tackle it head-on. Hardly a surprise, of course. I have presented many thought-experiments before, and the so-called answers were mostly evasions, trying to establish why the thought experiment should not have been presented in the first place.
The point of this particular question is very worthy of consideration. When it comes to taking someone else’s life, the “killing in self defense” is always presented as a morally acceptable exception to the “thou shalt not kill”. (Sometimes state sanctioned executions are presented as well, and also killing during a war. But these should be considered separately).
What Betterave questions, (and very properly, too!) is where does “self-defense” end? Is self-defense to be taken literally. or not? Do we have the “right” to protect our children? Do we have the right to protect our spouse? Our parents? Grandparents? Neighbors? Extended neightbors? Someone who is totally unrelated to us?
In all these scenarios there is common assumption: we are very reasonably sure that the life of the person(s) in question is in grave danger, and only we are in the position to do something about it. We can save that life (or lives), but only if we act decisively and we take the life of the “murderer-to-be”. This is the questoin, and it needs to be answered, and not evaded!