Arthur has never hated himself more at this moment.
As he lies in among your silken sheets, your chest rising and falling with soft snores as you lay curled into his chest, bare skin against bare skin, he hates himself
He was the crown prince of his kingdom. A mage with extraordinary power, he was beloved by his people and the court alike. That was until he was kidnapped away in the middle of the night, caught unaware, and spirited away to your kingdom, the enemy.
In the middle of a war, he was stolen from his chambers, dragged through hell and back in a night, and ended up dirty and disheveled on the stone floor of the throne room. Your father's face leering down at him from his throne, he was expecting to be tortured and killed. He wasn't expecting your father to give him to you, as a "birthday present" no less. A toy
Arthur had prepared himself to fight you, to incapacitate you at the earliest convenience and escape, he wasn't however prepared for you to be kind. Kind, sweet, quiet. It was almost insulting, he could so easily hurt you but he wouldn't, what would that bring him? Death most certainly. Then his plan changed, tempt you to fall in love with him. Gaining your affection means gaining freedom he does not currently have.
The king often talks in public, taunting him, of when his daughter will become with child—an heir for the empire. Arthur knows the reason he was given to you. A child born of his seed in your womb has the blood of both kingdoms, a child to reign both, means your father will win.
It has become more than a plot against you for Arthur, you're just so tender it's almost ridiculous. He hates himself for what he feels for you. He hates himself when he falls into your bed thrice or more daily. He hates himself when he kisses you in the gardens and he hates himself even more when he catches word of his father's armies failing without his help.
He hates himself because he's fallen in love with you