That's the thing that bugs me about the many games where you just cannot die, you just respawn or reload. So you kill, but actually your utterly safe from being killed. Oh sure, maybe your character doesn't know that he can't die. But what's going on, eh? Aren't we supposedly playing out the ancient moral imperative that if someone threatens your life, killing them in defence is okayz?
A: The games mechanics basically determine the fiction and those mechanics say you can't die. So that's part of the fiction. And so your playing out a character who cannot die, and so you can't at all be playing out the part of someone defending their life.
B: What, your not playing out that ancient moral imperative? So what the you playing out? Nihilism ala carte? If your going to say it's just a game, why pick one with depictions of killing in it over tetris, for example? Maybe fiction doesn't matter to you somehow - if so, okay, it's just a game but you mustn't be able to enjoy novels, which is sad for you. Or if you do, no, I don't think you have managed to turn your sense of fiction off like a light switch when it comes to games. You aught to consider that you are enjoying the fiction, and exactly what kind of fiction it is your enjoying. I bet some would think fiction full of consensual sex would be seen as RL morally wrong, yet this type of fiction, where the guy cannot die yet regularly kills others, as something just fine.
Here I'm basically against just thoughtlessly enjoying, for extended periods, killing while immune to death. If you want to consciously think about those elements when playing, yeah, I get that. But just relaxing into it and turning off your brain - like an Aesop's fable about the ant and the grasshopper that imparts a lesson, so to does the fiction of these games impart a lesson. Over and over, for as many hours that you play, that when your immune to death, killing is just fine.
Some will say it doesn't work that way, that somehow you have infinite free will to ignore that. I would be really interested in a scientific test where a game promotes something a bit more mundane, like eating a red icing cake type of cake over one with white icing. Don't tell the participants about it and run them through a week of gaming. Then send them into a room with actual cakes, see which ones get eaten the most.