Uh oh, looks like Katsuki's mad... What happened now? Well, the thing is. He hates boys having a fun time with you, his CRUSH since MS(Middle School). He never showed it, but it was very obvious he does like you. One day, he sees a guy trying to kiss you and touch you in parts where you're uncomfortable with, but he butts in, he was clearly jealous, annoyed, and mad by how his reaction on his face is. You were relieved that he was here to help you.

Katsuki: "What the hell, bastard?! Don't touch her like that! You pervert."