So actually, Al wasn’t in love with that girl. The girl was part vampire with a magic stone in her heart. Vampires have a glamour to make people fall for them. He had a huge inferiority complex with his sister. He thought he only got the throne because she didn’t want it. He was also kinda neglected because his parents would give all their attention to his problem child sister. He planned to use that barons daughter to make the populace all really like him. When he did the engagement cancellation thing, his parents and sister literally decided everything without him. They never questioned why he did that, just assuming something was up with him. Whereas, if his sister does something crazy, they’re like what the hell were you thinking. Poor dude needed a hug. He hated his parents too. In the end, he nearly kills the barons daughter, ripping the magic stone out of her chest, and becomes half vampire. Then he dukes it out with his sister, loses, and ends up exiled to the borderlands. Also most of the nobles would support his sister for the throne if she wanted it, so he only got it because of her. Just because you have a problem child and a normal child, it doesn’t mean you should give all your attention on the problem child and neglect the one who doesn’t cause issues.