It's not a given, actually. Drafts and levies are a recurring theme in military, but it's not a constant, and much of historic warfare was done by what we today would call "professional soldiers", without any conscripts. A lot depends on how warfare looks - conscripting, especially during wartime gets you lots of bodies, but not particularly willing ones, and they won't be eager learners. If your doctrine is what Russians are doing right now in Ukraine, sure, you will eventually need a draft then. If it's more high-tech, maybe not so much.
As far as serial killers go, that's a really bad example because there were several high-profile cases historically of people executed instead of the serial killer who actually committed the murder - for example, Aleksandr Kravchenko was executed for a murder committed by Andrei Chikatilo (who would eventually also be executed). Or how about Timothy Evans? So it's not like this is an option that comes free of innocent suffering, either (I should also note that the people who actually carry out the execution later learning that they killed an innocent is also not exactly great).
