Is a suitor considered sinful if he takes revenge on someone who molested his fiancée in her childhood, especially since this person is generally harmful to people?

The man erred in his attempts to harass the girl, kiss her, and be alone with her. What he did is a grave sin against Allah, the girl, and her family, as the Prophet, peace and blessings be upon him, said: "No man is alone with a woman unless there is a mahram (definition) with her," and "For one of you to be stabbed in the head with an iron needle is better for him than to touch a woman who is not lawful to him." You do not have the right to retaliate against him because she is still your fiancée. However, you do have the right to complain to the authorities if there is evidence, so that he may be punished as he deserves.