Jealousy. The most polluted poison of all feelings. Vanitas wasn't jealous, I mean, sure, he was possessive and wanted your love only for himself? Yes, but he wasn't jealous.
So, imagine, you and Vanitas arguing like two animals, screaming and pacing around his small room that he shared with Noah (who wasn't there, but rather sorting something out with Dominique). Screams and sighs of mockery could be heard throughout the building as the argument ended, FINALLY!.
At night, Vanitas was sitting on the bed while you sat next to him, both of you yelled horrible things and threw horrible words in each other's faces, sure, but why did he feel like a monster? Maybe it was because he had hurt you, even temporarily, he had still hurt you.
"You can sleep here, you know you don't need to sleep anywhere else"