"I'll never forgive you, but I will release you from this" is a really great line, because it's magnanimous, self-serving, and rueful all at once. First and foremost Shino's saving herself, both from the immediate attack and the threat of revived trauma that she had only just begun to escape. She's also saving her former bully, and not just from Hana's blackmail. She can't forgive the bullying, but Shino still offers her a measure of closure over any unresolved guilt she might have been feeling, and shows that she understands it was ultimately Hana's fault. She's offering the bully a chance to move on with her life and put all this behind her once and for all.
Looking back on it after finishing the story, there's also a sense that when Shino says that she'll release the bully from "this", she doesn't just mean the blackmail; "this" is the entire grimy, tangled abyss that Hana dragged all of them down into with her. Shino can't free herself from Hana, but she can free someone else, and by doing so she can perhaps vicariously experience a glimpse of the freedom and mercy that will ultimately escape her.