It's not saying that. Our original sunset illithid only said precisely what I quoted. If you're prepared to discuss things with it further to gain clarification rather than immediately killing it as soon as it finishes saying "I am sated for the evening and mean you no harm. I plan on moving out of your territory before I feed again.", then you are doing exactly what I said should be done;Since he did not make any such promises, no it does not.
"I'm going to move out of your territory before feeding again." is not even in sight of "I'm not going to harm those you're trying to protect".
I didn't quote Demonsbane for whimsy's sake. If it's saying that it will leave Kingdom A alone, but attack people in Kingdom B - it's not addressing "those you're trying to protect" at all.
You are talking and investigating the situation first.
Again, let's use the Orc: "I will not attack you. I leave your lands. Go kill the other humans. Okay?" *splick*
...when he was trying to negotiate and mistakenly assumed you were at war with Kingdom B?
There's a non-aggressive and unusual NPC. Jumping to conclusions and killing them based on that seems really really dodgy. Especially in the context of a campaign, where if the DM just wanted you to fight an illithid they would't bother with the bit about sunsets... but we'll ignore that for now.