When you really think about it, the Beast was just as bad as Gaston. The reason the Beast got cursed in the first place was because he was a selfish, vain man who only cared about looks, he left an old woman out in a storm because she was ugly! Gaston may have threatened to put Belle's father in a mental asylum, but the Beast locks Belle's father in a dungeon and then forces Belle to stay so she can fall in love with him and break the spell! That's just as bad if not worse than Gaston! Belle only gave him a chance because he saved her life after he scared her out of the castle, and only then did he change. Maybe if Belle gave Gaston the same chance he would have changed? We'll never know. However, Gaston threw Belle's book in a puddle whereas the Beast gave her a library. She made the right choice.
This is of course referring to the Disney version, I'm fully aware that there are other versions which are messed up in their own way.