Sadeas was a rat, but killing him didn't seem in character for Adolin, he was always about duels and challenges etc.So ya I'm not supportive of it. Sadeas needed to die, but if it was going to be him being knifed in a hall by someone it shouldn't of been Adolin. If Adolin was going to kill him it should of been in a fight either a duel (unlikely) or in battle.
The imagery of his white horse being killed was foreshadowing I think. Adolin has started on a dark path, or at least a grey one. Do I think he is going to become a villain? probably not, but I think he is going to be confused, maybe more so than Moash was at the end of the book.