then more making her become his woman. the reason his brother get killed because he is the fiance of seraphina. once he gone, the prince can engage to her. by making seraphina his woman, the target will shift from his brother to him. he is stronger as he is genius mage, so that is far better option.
the prince want her, anyone that tied to her is a target to be eliminated. what point of killing his brother when MC is engaged to sera? not like sera become free and ready to be seduced.
He become Seraphina is the best move as he aggro the threat on him,.and at the same time he can protect sera as she close to him. that why i said MC is idiot. to keep his brother safe while engaged to sera, he need to always alert and kill prince, velary and anyone that tried to use sera. He deliberately choose the hardest path because he is too coward to confront his feeling.