Hmm... I believe even though Joon Woo's actions may be considered stalkerish, he is different from the other stalker that he actually moves more for the sake of Sung Ah than himself, even telling her to get together with class rep, giving up on her, but still hoping that maybe someday he'll get a chance or something. He most likely won't really try to harm Sung Ah and Class Rep's relationship if they get together, seeing from his choices back then (the one with concerning Taeyang might beg to differ, but that one's arguable). He was willing to step down too.
I cannot stick up for class rep here, he's clearly provoking Joon Woo, probably because he can't get Sung Ah for himself, so Joon Woo getting fighty like that isn't 100% his fault. He's clearly forcing his judgement on Joon Woo as far as I can see. Whether he's self-righteous or not, whether he himself is a stalker or he was just able to deduce it due to him missing that day and showing up the next day bruised, we have yet to see that. What we can all say about him though is, something's off about him, and he may well be dangerous.
Edit: Hamin is frickin great! And she's the center girl in the next cover, so who knows if Junu's really gonna end up with Sung Ah? At the very least, she would be a nice partner for Joon Woo if the author decides that she'll be a very close confidant. Haven't read the raws.