I'd lean on the sorcery aspect. Have the King be possessed, or brain-washed, or replaced (method unknown to the hero), and thus acting out of character. Have the hero realize something's wrong, and the King has to be stopped, but is unable to do it himself.
Then a mysterious assassin arrives in the middle of the battle, and the hero watches as the King is slain. Others witness his inaction, as he lets the assassin escape, and word spreads of his cowardice/betrayal.
The next in line doesn't believe the hero capable of either, but can't refute the testimony of the others. None of the clan's enemies ever claimed the kill for themselves, so the mystery remains. With no other alternative, the hero is banished.
After a period of time where he questions his own sanity and sense of guilt, the hero realizes he needs closure, and sets out to find the one person that might have the answers he needs: the mysterious assassin.