THEORY!:
You know what l know how this is going, painting the MC in the bad light, but what if he wasn't the one who recorded the video of the daughter being harassed/what if he wasn't the harasser.
I don't think it would be too far fetched to believe that the only reason he is going along with the bullying is to record/ gather enough evidence against the bullies (since we were shown a hidden camera in his tie, something that wouldn't really be necessary to show since everything is hinting to him stealing the daughter's phone and recording 'that video'. Like they plant seed the doubt in our mind the very moment he goes to give the daughter her phone just for her to later find out that there was a video of her being harassed in it). Like every time we saw the daughter getting bullied he would just stand there doing nothing, couldn't he simply be doing that to record the acts of those girls?? If he were to involve himself as soon as he saw the bullying there is a high chance the bullies would simply get off the hook by them only apologising, causing in turn them bullying the daughter even more but this time being even more carefu,l to make the sure the teachers won't find out (its possible that if the word of the bullying also spread the girl's father may start treating her poorly because she in his opinion would've become the type of weak person he despised. So it's entirely possible MC is trying to avoid that). Another thing if he was doing this purely for revenge or out of amusement and just to see his past bully's daughter suffering the same way he did wouldn't he show some of those feeling on his face? (Like yea maybe he's really good at covering up his emotions but if the guy is petty enough to bully a young girl (and possibly ruin her life the same way his life was ruined) wouldn't he atleast show a smile & wouldn't stay so stone faced looking completely dead inside??).
Another thing when the straight hair bully girl showed the friend 'the video' it didn't seem like was sent to her by someone as she seemed awfully calm (cuz even if she was bullying the daughter there would atleast be a little surprise on her face since she just got sent a video of someone getting harassed by an anonymous person), isn't it possible that either her or one of the other bullies recorded it? Specially when we have been told that the bully girls did indeed visit her in the infirmary (and later just gave the MC the phone, cuz apparently "he's in on the bullying?").
(EDIT: l went back and reread this chapter & remember how the daughter told him that he always smiles when he lies, well when the straight haired bully girl talks to him about him "taking their side" he smiles after the girl leaves, possibly hinting that he is lying about simply going along with them/is putting a fake facade of being with the bullies.
I honestly really hope l'm not looking too into this manga just for it to simply have a shitty twist of the main guy being sadist asshole. And it it would also be really annoying if this comment aged badly after me going wayy too into the story.)
So l'm going to wait before l deem the main guy shitty since l doubt that this manga is going to end this way, specially when the first few chapters made us feel sympathy for him just for the manga to put a bad twist and make him the bad guy. (If it does then that would be a big yikes)