When using the /kill command in The End and The Nether, it will kill mobs that are in the overworld.
To Reproduce:
Go into the end or the nether and type the command:
/kill @e[type=!Player]
The pictures show that the things killed are items and mobs that are found in the overworld. When returning to the overworld, everything is killed.
It kills entities on the entire world. This is intended.