Rule number 1 as an assassin, always kill the person/people who are threats ASAP.
Rule number 2 make sure that they are 100% dead, even if you have to remove their head and/or heart(s).
Rule number 3 double check rules 1 and 2.
But seriously if the story is gonna be about an assassin they really should just kill all of their potential enemies and threats right away, assassins need to keep those things to a minimum or they can't do their jobs. I see no need for him to think about whether or not to kill the evil king and his probably just as evil if not more evil daughter. He probably could've extorted the information out of the king before he killed him, since he is probably at least 12 times stronger than him (1200 attack vs 100 the average max). Still this could be interesting or it could fall flat, I can't tell just yet, but even if it did I would probably keep reading for a while.