I had a chicken coop and someone got to the top and killed them all why do they get to kill? I think they should fix this promblem.
You can turn off passive animal damage in /settings, so only trusted players would be able to harm them.
That is not possible. If you do not trust the player enough to know they will not harm your pets, then simply don't /trust them.
If your island is locked, (/lock) and your islands settings are set correctly, and you trust only players you know, as Fran mentioned above, then this shouldn’t be an issue anymore. Which server is this on? I can spawn you a few chickens in to make up for what you’ve lost.