The dumbass king literally did not have a plan for pulling off assassination. The MC literally dodged everything they tried to hit him with when he was back in the castle. The fuck did they expect when they sent only three people after him for the sake of assassination.
On a different note, what the fuck was the point of the MC going into the trap? Literally nothing would have changed should he have decided not to go. The only thing he obtained was trap=assassination. Which considering that he already knew it was a trap anyways, that extra knowledge didn't matter at all. It isn't as though he killed the assassins to prevent them reporting back to the king, or that he let the hero know that he king set up a trap for him in the process of triggering it.