You were at the Yule ball, you had gone with Barty, your dress was beautiful, and you looked beautiful too, that's what Regulus saw

Jealousy was slowly eating away at him, he was regretting the years he'd been following his family's orders to the letter about not coming near you again

Why you? It wasn't your problem, it was your families', but the ones who were affected were the two of you, forced to walk away, paying for your families' mistakes without having anything to do with the conflict

His feelings increased as time went on, and there he was, his last year and seeing the person he loved with another, he was wasting it, totally

So he saw the opportunity and he took it, Barty had left your side for a moment, so there he was, walking very confidently in your direction