Hmm?
On the one hand, I could see this as just a case where the girl was just a petty bully and this turns into a wholesome romance by the end. Which I honestly think is what the goal is going to end up being.
On the other, I've seen enough twisted stuff and real life messed up stories, that I could also believe that the girl is unredeemable and absolutely deserves all the misery she gets coming her way even though she's behaving like this, because we're just seeing a very tiny window of her life here which is where she appears as weak and as frail and as regretful as she can be. But for all we know she's very selfish, manipulative, two faced, and her bullying that she inflicted involved unforgivable acts to the level of which you would see her as one of the bullies in "Buta no Fukushuu / Revenge of the Pig".
And really, until we know why the MC wants revenge and what she did that led up to this, there really is no way of knowing how to go one way or the other. And while some may suggest that it's heartless to be so on guard to a crying young girl. Again, I've seen enough messed up stories and real life circumstances to know that just because someone is crying and trying to get your sympathy doesn't mean that they are innocent or aren't the aggressor and have done far worse and are attempting to do far worse through gaining your sympathy.
Of course, ideally, strive to still be a good person, but make sure you can maintain enough clarity to be able to take in new information and readjust your evaluation of the situation.
Alternatively, his revenge could just be something as simple as just showering her with as much love as possible that she forever regrets being his bully to begin with. Which can be done without resolving the core issue and never dismantling the nugget of doubt she has. It's not a heartwarming course of action and definitely would require a psychological tag at that point because if we had the mental monologue of the MC being a sociopath who fully understood what he was doing and not at all being emotionally attached to her, that would also be one way of going with this premise.