In terms of why he was an insufferable jerk, I can see. If we're thinking in legitimate terms here, the Beast lost his mother and his father was a manipulative a-hole. The trauma of losing his mother and his hatred towards his father rendered the same insufferable personality in himself. It only grew worse when the witch cursed him to live in isolation. He's been stripped of everything he has and the only way he knows how to deal with trauma is through anger. By no means does it justify, it's just relatable.
As for Belle, that's a tough question. It's dangerous and I would never condone such things. As to why Disney, the overlord company of all things cute, is perpetuating this is beyond me. It seems that they want to preach a message of individualism and feminism, but all the while promoting unhealthy expressions of love. And for what? A cute love story? A girl falling in love with a douchebag? An abuser? Nothing in Belle's backstory reveals to me why she would be attracted to such a character. I mean, he changes, but still. The one thing that perpetuates her change of mind is his coming to her help. All of a sudden everything is ok? I don't know. It's iffy.