As you know if your main is Sebille or if Sebille is in your party you need an incredibly difficult persuasion check to avoid a fight with Zaleskar, but the real problem is that if you kill him he does not drop Shadow's Eye, which makes no sense as it's a powerful quest item for Ifan and the death of Zaleskar will make you miss it and break his questline if Ifan is not in your party when you talk to him.
As a rule of thumb key NPCs drop notes (or limbs for elf character to eat) so if they die the quest progression is not compromised, but in this case it does not happen.
You cannot know this beforehand and not everyone put points in persuasion on the main character, I had them on the red prince, but if Sebille is your main Zaleskar only wants to talk to her, which make the use of persuasion you buffed on other companions impossible. So you cannot avoid the fight and if Ifan is not with you then you are screwed. I only noticed this few hours after I killed Zaleskar and I could not reload, which turned into a massive pain in the ass by losing Shadow's Eye and screwing Ifan questline for the whole first act..
