I was reading a review of 13th Age to see how it differs/evolves from 4e.
So, in 3e you could choose to do non-lethal damage, and folks could knock each other out without killing each other.
As a mechanic, make the inverted and default. Players have to declare their intent to kill.
I like this, I feel it works to genre. Bloodsucker going for the kill? Loose humanity. Suppressing the “savages”? Alignment shift!
This falls into a category of mechanics I consider tonal, similar to the Veil and off screen action.