What actually happens to dead colonist? I've killed hundreds, perhaps thousands and am wondering if the game had a cemetery or body storage area, maybe that could be a clue that your headed for disaster before it occurs and its Game Over.

There's a negative penalty depending on the cause of death.
There's also a mod from a year ago that added a cemetery, but I'm not sure if it still works.
https://www.nexusmods.com/survivingmars/mods/37