From a rational perspective, what is wrong with saving yourself? Or is it simply an emotional decision?
Well, I imagine the guilt would be constant burden on me. Something that at least drags my life down and at worst drives me to self-harm. The life of the other person, who survives (and, if it was not self-defense, did not need to kill to do so and is possibly fully unaware of the situation), would be unmarred by guilt and therefore better in quality. Self-defense is more easily rationalized, as is killing someone who is the worst king of terrible (i.e. a Bundy or Dahmer) or someone I know poses a continuing threat to others.
The issue of the guilt that weighs on me is not wholly rational but I am not certain I can escape it. Survivor's guilt is often called irrational even by those who experience it, but they still feel it.
I would feel guilt for killing the baby but the fact that the baby dies and several others live means that ultimately my guilt and the baby's death are less than the cost of everyone present's life. To me, not killing the baby may be a selfish action.
Well, we waver into generalities. Maybe the baby I kill is going to grow up to be a mass murderer or genocidal maniac. For the most part, that isn't true. For the most part, people would like to live. I can only make the decision based on likely input or results.
I guess if I were to kill Martin Luther King to save myself, Stalin, Charles Manson and John Wayne Gacy, no, I wouldn't do it.
We could get infinitely semantical, but I don't see the point. If I want to answer "but what if"s ad nauseum, I'll hang out with a 4-year-old.
Of course it depends on the specifics, which is why I take hypothetical situations to be "in general" or "to the best of your assumptions, based on what you see in the world and people around you." I don't arbitrarily assign significance, good or bad, to anyone in the scenario except those explictily stated to be killers or saints.
If I did, I'd have to look at the vague situation "Kill one person or ten" as "well, maybe all ten are suicidal or evil, so I guess save one"