Hasn't there been a pretty big implication that he struggles controlling himself?
I thought It was implied at one point he accidentally killed Kethric's daughter?
eh? when?
I sort of recall some theory about that a long time ago, but I'm not sure there was ever anything solid there. I think the speculation was that maybe Ketheric "fell" because something bad happened to him... he had a daughter... maybe his daughter died and that's why he fell? I believe a piece of text specifically mentioned that he "fell." And in earlier patches, there was more information about ketheric sitting around, if I recall correctly. Anyway. It's been a while since I looked at anything about Ketheric's daughter, but isn't she:
Isobel? Who we can meet in the Shadow Lands with Jaheira.
As far as Halsin not being able to control himself, he tells us that in EA when he offers to help fight the goblins in the temple. He warns, if I go with you then I won't be able to control myself and we'll likely have to kill every goblin in this place. Or something like that, paraphrased obviously.