@Rachelharris31 Aha, I see.
The Death toll could also be caused by the fact that you are putting to much pressure onto the game core by playing with so many Sims and that this is the game's way of restoring a kind of balance in order to control the rest of your NPC-s.
I wonder what would happen if you restore the amount of Sims back to the default of 200?
In the situation you don't want to go back to the default, the option that was suggested by
@kreatora should suit you better.
But than probably the game would still want to restore a sort of balance so you could run into some other glitches or deaths when you keep on playing with 250 Sims.