It's a ridiculous hypothetical
It's not ridiculous and it's not hypothetical. Several studies have shown a positive correlation between access to legal abortions and reduced crime rates (pretty obvious when you think about it - if a child isn't wanted then it's less likely to get a good upbringing). Certain mental disorders such as Psychopathy and Manic-Depression are known to have strong genetic components. If fetuses with these traits were eliminated then murder rates would drop dramatically.
However, some people consider all abortion to be murder, and would force a mother to carry an unwanted fetus to term no matter what the circumstances. And yet, these same people may enthusiastically support executing that hapless individual as an adult
after he has committed the virtually inevitable capital crime. Quite a few even seem to take pleasure in the idea of having a personal hand in his execution. Of course it's only a small step from there to killing an abortion doctor because he's a 'murderer'.
I don't see where this has anything to do with punishment after the fact
Which do you think is better - eliminating potential murderers before they become actual people - or waiting until they kill somebody and
then getting all upset about it?
If you knew with 100% certainty that some man, woman, child or fetus would go on to intentionally kill one or more people in a criminal act and the ONLY possible way of stopping them was to kill them, wouldn't you do it?
Would you do it if this person was going to kill you? Or your loved ones? Your friend? A neighbor? A stranger? Someone you dislike? I would say yes to all of those.
Under those hypothetical conditions, killing the perp before he can commit murder is an obvious choice (though it may not always be the best course of action). But if you were unable to stop him, would that justify killing him
after the fact?
What justification can there be for using the Death Penalty?
1. Teaches him not to do it again. Pointless 'lesson' when he is now dead!
2. Prevents him from doing it again. Life in prison has the same effect, but without the extra killing.
3. Gives closure to friends and relatives of the victim(s). Life in prison and forgotten about is easier on them than 'Celebrity Death Row'.
4. We get off on it. Finally, a valid reason to keep the Death Penalty!