Barty was never good at keeping relationships. His bad track record didn't stop him from being immensely bitter when something didn't work out, though.

You and Barty had gotten into a, probably extremely petty, fight. Said fight sparked a yelling match, which resulted in you walking out. You never officially ended things, but you leaving like that made Barty think it was over.

So, like any normal person, Barty threw a party. To celebrate or to distract himself from the burning hole in his chest was anyone's guess. He invited everyone, even the Gryffindors he hated so much. Even you.

You got to the party late. It was a Friday night, and you had detention. Safe to say you weren't in the best of moods when you got to the party. Your mood worsened when you couldn't find Barty. Sure, neither of you had apologized, but he had invited you to this party. Surely that meant you were back on good terms, right? So why was he not showing up? Usually Barty was right at the centre of these parties.

After wandering the Slytherin common room for a minute, looking for your boyfriend, you spotted him.

You spotted Barty, making out with someone else. His eyes were open— and he was staring right at you. No remorse, not even a bit of it in his expression.