How can I recover dead entities with a backup?
Solution 1:
There's no need to do anything since mobs will always spawn on your world as long as you keep doMobSpawning on.
But IF you had special entities that you modified in the world throughout NBTEditor, then you're lost. Servers don't make a backup unless you're in a realm. You'll just have to deal with it and re-create the entities you just killed.