He overpowers his opponent a lot. Everyone is surprised, but he wants to finish it with an amazing techinque, to show Demon King that he is grateful to him and also prove himself to him. He succeeds, and he uses a technique of Demon King's. After that, his parents try to intervene in the fight. He basically spills the beans by saying that he only wanted them to acknowledge him, and accept that he also worked hard. That he didnt wanted to be a hero's child, if that meant people would not see what he actually is. He hits everybody around him. He pushes his mom and the princess, and he also punches his dad in the face. Then, he just runs away. Maid is also very regretful, because she freaked out when he used Demon King's technique. Then Princess, Mom and Maid go on a journey to find MC. Also dad tries to find him in his own way. Everyone is saddened by everything and the family is incredibly regretful.
But this is the great part, because MC is actually free, and he meets amazing people. Man, I hope you decide to read the novel. I really like it.