These posts kinda miss the point of thought experiments like this. The rules simply are as they are, use magic to explain it if you want. The point is to consider whether killing the kid would be worth the outcome promised.
Except this isn't an experiment. What real world application would this have? I know plenty of people that would act for the "greater good" in a certain situation, but this is basically taking it to illogical extremes.
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TheRock ~ Slow dramatic zoom-pan. Doesn't phase the hooded man.