[SixOfSpades]

The dialogue does not revolve around whether or not you're going to attack/fight Sarevok, but whether or not you're going to turn into the Slayer to do so. A Paladin would not assume the mantle of the Lord of Murder simply to kill one pesky half-brother.

(The weird thing is, you can say that you won't use the Slayer ability, but then do so anyway--or conversely, say that you will, and then don't. And the game goes by what you said rather than what you did.)