I think it might be a little deeper than just an edgy revenge manga, because MC basically becomes like Sada when he causes suffering a response to his own suffering. "If you're having a rough time, does that make it okay to bully people?" (page 18).
I was somewhat expecting MC, upon knowing that Sada is bullying him for a (flimsy) reason, to somewhat turn the other cheek, but I guess that's tough to ask of the victim so I'm not surprised at his reaction. That's superogatory.
I don't see why everyone dislikes the girl; it's obviously not enough, but any harm from a weak apology stems from her ignorance, not her malice. "No good deed goes unpunished."
(I think if any of them are more deserving of punishment it would be Sada's friends. Not in terms of causality, since if Sada wasn't there the bullying wouldn't have happened or be a lot less, but in the sense that Sada is doing it out of anger, whereas his friends find genuine joy in being malice. Sada reminds me a little of Punpun when (rot13 spoiler) junpxf n thl bire gur urnq jvgu n jbbqra ornz sbe qvfyvxvat uvf sevraqf znatn (bofgrafvoyl vg'f orpnhfr gur thl jnf yvgrevat).
(The fact that Sada is the root cause of MC's bullying more than his friends I think paradoxically makes him less guilty, since Sada "needs" it, whereas for his friends MC's suffering is just a fun diversion in their lives.))