Perhaps if I looked in the Population section first I'd have figured it out for myself. Sorry about that!

I'm still confused. I don't see a "Population" section in my UI (just Campaign, Hunt, Showdown, Timeline). There's the Survivors board, but I dont see a way to kill a Survivor, only delete them (which doesnt update the Dead Survivors number...

The buttons for a campaign that is underway should be (from top to bottom):
Setup
Hunt
Showdown
Timeline
Population

Clicking the Population button will bring up some UI where you can tick a "Dead" box for a given survivor. (I suppose they're not really a survivor at that point, but hey....)

@dawsonsmythe Assuming you're using the latest version, look at the "SURVIVORS" board at the bottom of the table. Each survivor has a "Delete" button.

I generally check the box for 'dead', return the survivor to the settlement, and then filter my survivors by not showing the 'dead' ones. That way if I ever want to go back to a survivor, I can! (Light Spoiler type hint - there is a slight chance you 'might' need dead survivor info for a part or two in the game of the game).