drop this because it's pro bullying
In this manga, the author clearly writes bullying as something so horrible and wrong that the bully continues to punish herself to this day and she is never truly forgiven, not even by herself.
That is exactly my problem, bullies in real life never feel this kind of remorse, on the contrary they continue to live in a natural way and in the worst case they are happy and successful people who trampled on the lives of many just to feel better. The manga makes it look like they are really sensitive people, which they are not. Anyone can feel remorse for their bad actions, that being part of the plot so that we feel sorry for a character who was a bullie is pro-bullying.
Besides the other comments which reacted to this, I think you're ignoring two important things.
1) "Bully" is not a category of person that is the same across all age groups, social classes, genders, etc. People bully, and people are different. There are certainly bullies who do not feel this kind of remorse, and there are those that do. This manga depicts one particular person who bullied someone and felt remorse. It's not a statement on all bullies everywhere.
2) They were kids. Really young kids. I give a lot of leeway to kids for doing stupid things and then growing and learning from their mistakes. Fifth graders in Japan have an average age of 10. I don't think I would take any 10 year old's personality as their final personality or take it as a given that how they act at 10 is how they will act for the rest of their lives. I think it's silly to assume that.