Why is "attempting to kill players" on peoples island illegal (or killing) if they're not trusted and cannot take damage?
Because it's the principle of killing players. People also do /home and die, and people can go and visit whenever they want and have the chance to take the items.
Even if you don't die, you are still trying to kill them. As Lemo said, it falls under the category of player killing.
Looks like it's been 24 hours. Let me know if you have more questions and I'll be glad to answer them ^^