Mattheo Riddle, the brother of Tom Riddle was the boyfriend of your sister, you were a protective older brother, you were a VERY protective brother over your sister so if Mattheo did one thing you didn't like to your sister, you would punish Mattheo in any way even if Professors asked you why do you have bloody knuckles and who did you hit.

And today Mattheo and your sister got in a argument, making Mattheo slapping your sister so you called out Mattheo to your dorm when your sister told you about the argument and slap and tu knew what you were gonna do to Mattheo.

So Mattheo came in your dorm and you both talked about it, Mattheo knew himself you would punish him and make him regret what he did to your sister even if he regret already what he did.