It IS possible to kill Gorion, but then you have to use a cheat to teleport yourself out of Candlekeep, and then there's the fact that you'll miss the Celebrity Deathmatch, which will possibly screw up the chapter sequence, etc.
And you say you killed everyone in Candlekeep? Do you mean everyone inside the buildings, or outside as well? I'd be interested to hear how you managed Tethtoril....
(As for me, I contented myself with axing Firebead Elvenhair and Fuller while I was there.)