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.
This isnt about utopianism at all.
And no, you can't justify death penalty because some people "get out on a technicality". Innocent people end up in jail on technicality too.
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.