You didn't kill Isobel. You awaken with a churning stomach. The sight of Astarion resting is present as your butler appears...

"He is so afraid. So, so afraid. Of everyone, besides you, who he ought to fear most. You could do so much better..."

He goes on about the idea of killing Astarion, killing who you care for most.

"You like him for more than his looks, but he will never believe that. Why not make him a pretty corpse?"

Your companion rests blissfully, without a fear in the world...