I think that this will vary by game and by group.
Interestingly enough, my players are far more likely to simply kill prisoners in D&D where they play heroes than they are to kill prisoners in our Blades in the Dark game where they’re actively playing a group of criminals.
I noticed this trend and there are a few reasons for it I think. The big one is alignment. Alignment seems to me mostly to serve the purpose of making murder acceptable. I don’t think most Good creatures would slay prisoners as readily as most Good D&D characters appear to. But if hose creatures are Evil, then for some reason it’s fine and takes no psychological toll.
The second is the othering of most enemies. Orcs and gnolls and ogres are all sufficiently removed from humanity that it’s easy to view them solely as monsters. Which is absolutely fine if you want to play that way, but I think it bleeds through even if you wanted a more nuanced approach.
In Blades, all the enemies the PCs face are humans, and none have an alignment. There’s no label that makes it okay to simply slaughter them without concern. The PCs don’t have an alignment, either, so I think the players actually give more thought to what individual actions may mean.
The game also makes killing have potential repercussions. It can reault in the Crew taking more Heat from the fallout of a Score. It can result in the creation of Ghosts, which is dangerous for everyone living in the city. Also, the Spirit Wardens are like special ghost cops that are summoned whenever someone dies in the city so that they can prevent the creation of ghosts. All of this serves to make killing a significant decision for PCs to make.
Again, I’m sure many folks simply like to play to have fun and not worry about the moral implications or psychological impact of murder on a person, and that’s fine.
But I think that’s a rather different thing from trying to apply this simplified view to the real world, or to cite true events in history as support of it.