I used the /kill @e command when I was in the Nether, to kill varying mobs that kept bothering the construction I was doing, and it killed all my animals in the Overworld. This was extremely frustrating because I spent a lot of time choosing the animals’ names, colors, and placement.

Screenshots from top to bottom because they don’t make much sense:
1.) I made an example of what happens when using /kill in Nether with random animals. This was made far from my base.
2.) I used /kill in Nether. You can see the list of animals that die
3.) I went back to the Overworld. Strangely, the example animals survived, but the ones at my base did not.
4.) I went back to my base. My animals are dead again. Please, I don’t want this to happen again. That was the third time.