Designing games with high-mortality

[Breccia]

Do you just assume that the player of a dead character just leaves (assuming no replacement characters)

Only in a one-shot.

I know you're trying to run a horror game, and the fear of meaningful death/loss is pretty common. But for good gameplay, sometimes you have to sacrifice some realism. Nobody wants to be Boromir'd and watch the rest of the campaign while having nothing to do. They'll leave.

One option is to have dead PCs start playing villains or monsters.