At this very moment, the beast must have been heart broken. His beloved girl is gone. But I also have this feeling that smile must have spread over his face because of this right choice.
Appearances are often deceptive, and that things are different from what they appear to be. The beast found himself back. He knew he must grow up and try not to be bad-tempered and severe to others. He felt empathy with Belle. So he let her go. Enable to use every chance to make her happy, the beast must have known this is the greatness of love. Love is not possession. It is composed of respect, equality, sharing and giving.