To not act on them shows strength of will.
Maybe in a few cases, but in the vast majority of cases the bullied person simply doesn't have what it takes to act upon their dark wishes. That's not the same as sparing the bully. Facing the bully and making them suffer is not even necessarily the worst part of what's holding the victims back, it's fearing the legal consequences. Bullying is most of the time insidious, subtle, and slow, and might have the bully suffer a lecture from the teacher, but that's it. However, if the victim wants to pay everything back, it would most likely need to be swift, which means it would need to be drastic to have any meaning. Thus it's probably illegal as well.
If the victim is truly broken, they might do it nonetheless, but quite often they also take their own life afterwards. Otherwise the victim would need to ask themselves if it's worth it to first suffer years of bullying and then suffer years in prison, possibly even paying compensation money to the bully, depending on their wounds/material losses. None of this has got anything to do with strength of will.
In most realistic cases strength of will would be simply to overcome the bullied past and become a successful, happy person who dies of old age with no regrets worth mentioning. I'm not sure if becoming a successful edgelord isekai revenge novel/manga author fulfills this condition. It doesn't really sound like being able to leave it all behind.