The line of reasoning really only works if you are talking about yourself. If we assume all lives are of equal value (which is a big assumption but not without precedent), then killing your would-be murderer is a wash, but it does raise the question; why should you be the one to live? And the justification seems like it must be based on either 1) a belief that the transgression of attempted murder justifies self-defence, or 2) that the Self is simply more important than the Other.

When a third party becomes involved you only need to rely on option 1. You are still probably acting out of “selfish” reasons in this case, however; I’d rather save my child than preserve the life of a murderer, but that is simply because my child’s life is more important to me than that of a murderer, regardless of moral justification.

The questions about self-centeredness get more interesting in life boat scenarios, where you have to choose between equally innocent parties.