We think about that all the time but drafts will be a fact of life as long as war exists and many countries have compulsory service. Even in the case of the volunteer, it is OK for a state to kill a solider who volunteered but not a serial killer?
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).
1. The state isn't sending the soldier to die, it's sending them to fight. They may die, but dying isn't the purpose. The state would very much prefer them to not die, in fact.
2. Nothing bad happens if we just don't kill the serial killer.
The "state" does not have a preference, states are conflicted by nature. Probably would not take much effort to find examples of serial killers who were not killed doing something bad, killing another inmate, killing a guard, getting out on a technicality, etc, etc, etc. Yes, if we lived in a utopia things would be perfect and you would not be ignoring the point.
My definition of utopia allows for disagreement, my issue was with his methods, avoiding the original point and not the disagreement. I never tried to justify anything, at most I expressed a preference for killing serial killers over having wars.
Why? Because killing serial killers is good, and having war is bad?
I think the person you were responding to had a more utilitarian view. When war happens, the death of conscripted young people is unavoidable. It's kill or be killed. We prefer wars not to happen, and people not to die, but it's not a choice.
Killing a serial killer who has already been apprehended is a choice. And it's here that we can actually start weighting options and see what is better for the society, bringing up possibilities of wrongful convictions, Blackstone's ratios and second order effects of the death penalty.
I made no such judgement, just sort of but not really implied that I would rather serial killers die than random people and then admitted it in a purely intellectual pie in the sky sort of way—killing serial killers will not end war. Plenty of examples in history when a draft was used in times other than kill or be killed, when it was political. It is ok for the state to kill its citizens for political reasons but only when those political reasons are war? At what point does civil unrest become civil war? When that line is crossed the killings suddenly become OK? What if it were treason instead of murder and the execution of that traitor could help mitigate civil war and far more death? That comment opened a massive can of worms, it is more complicated than ideals and what we would like.
States killing 18 year olds who got drafted and never did a thing wrong in their short lives is more acceptable than states killing serial killers?