To me, deliberate hurtfulness is one of the very few things I cannot forgive under any circumstances, and bullying falls squarely in this category. If a bully later shapes up and becomes human I will acknowledge their growth, but I still can't bring myself to forgive the damage they've done.

It's like the parable of the plate: Smash a plate on the ground, then tell it you're sorry. The apology does not make the plate whole.