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Maybe Snicket Warning Label (generic "upcoming plot developments are bad" trope) in lieu of anything more specific.
There's also Ominous Save Prompt.
I'm not sure if it exists. There aren't many games that allow the players the level of freedom to just swing at everyone and deal them damage.
Space Battles Dot Com fanworks (unnoficial) index in my Sandbox.
^^ I think its kind of the opposite of that as it gives you a warning after you kill the guy, making it an Ominous reload prompt if anything.
Unwinnable by Design covers games that allow the player to do things that will make the game unwinnable.
A type of Anti-Frustration Feature that separates the "tough" and "nasty" forms of Unwinnable by Design from the "Cruel" form- it will totally let you kill said NPC, but it will make it clear immediately afterward that you've done something that makes the game unwinnable rather than 6 hours from now when you get to the quest where you need to talk to said dead NPC. (something for which early-gen adventure games and open-world RPGs were notorious)
The tough and nasty forms are separated by how obvious it is that you probably shouldn't kill that NPC in the first place (say, they have a name or a unique outfit)
Edited by Scorpion451
If the warning comes immediately after you declare your intention of killing that NPC but immediately before you actually start to kill that NPC, that's probably Are You Sure You Want to Do That?
If not for this anchor I'd be dancing between the stars. At least I can try to write better vampire stories than Twilight.
Is there a trope for when a game warns you if you kill some plot-critical character rather than just giving a Non Standard Gameover? Example: IN Elder Scrolls III Morrowind if you kill some character crucial to the main quest you'll get a warning about how the "prophecy" has been broken and the main quest is now unwinnable, presumably so you can reload if you didn't mean to do that.