This discussion turned out to be a very interesting read. This debate on ethics, official ratings of immoral actions and how individuals see those actions should be held in schools, with children. Before they get a chance to either get a PTSD or become indifferent. I edited the OP to warn people about this.
I had to search up what Hors de combat means. For everyone else, this is what Wikipedia says:
It is a French (meaning 'out of combat') term used in the laws of war to refer to persons who are incapable of performing their combat duties during war, thus generally not treated as active combatants but rather protected persons.
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@Jmanis You're very much right that it's subjective. I'm about to share my interpretation in hopes of clearing up confusion.
Instead of being combat capable, I'd draw a strict line between an aware and an unaware person. An aware person faced with imminent death starts panicking. In my personal opinion, any death that includes fear is a bad way to go (regardless of pain).
In that regard, a stealth kill with a blow to the head and killing an unconscious person is the same. They die without any fear or panic.
On the other hand, a stealth kill where the victim suddenly feels sharp pain and sees a blade "growing" from their chest is the same as shooting someone or cutting their head off. There are a few VERY scary and painful moments before death and I consider that a bad way to go.
If the kill is not made from stealth, be they hors de combat or not, it's a bad way to go and carries the same weight in my view. For example, a tied up hostage knowing it's their turn is the same as an unarmored, unshielded scientist armed with a weak weapon, trying to kill a player in Dominator suit wielding a kinetic primary. In both situations, the victim knows they'll soon be dead.
If I had to be on either end of "the transaction", I'd choose the incapacitation first to be more morally correct. This is why I made the topic.
I'd still prefer to leave the NPCs alive, be they conscious or not. If they're trapped or tied up or frozen in fear or rendered no threat in whatever other way, it'd feel better than just having to kill them.