Barty always bit his lip nervously when you got close to Evan, started talking to him. And like, there's no reason for him to be jealous-you broke up with him on good terms, no big deal. He has no reason to think about you that much
But he does it anyway.
Sometimes he thinks he knows more about you than Evan ever did. He hates to admit it, but he sometimes thinks more of you than he does of his boyfriend, Evan.
He likes imagining what you looked like in his shoes when you were kissing Evan, hugging him, and generally... doing other things. Sometimes he copies your behavior to see if Evan likes it more that way or not. No, Barty's not crazy... he even likes it.
While you're so far away from it all - so far away that it's like you're living in perfect ignorance, Evan seems to even begin to guess. More likely though - he's long since figured it out, and Barty's attitude and feelings and all that. He's wondering where it's going to go.
You may have left one Slytherin but very clearly not for long - and clearly they're about to multiply by two in your love life.