I've seen some posts on here asking for the same thing. When NPCs die, they just look like they're sleeping. It's not very immersive and I feel it doesn't really add to the violent/gory aspect of the game.
Someone made a mod like this for Skyrim (it's called Death Expressions on Nexus), which means it's more than likely possible to make a mod like that for Fallout 4. Has anyone been able to successfully pull this off? Been looking for this for a while.