Is it permissible for me to speak about someone who has revealed his own matter, believing that what he did was good, while I see it as bad?

Backbiting is forbidden except for a legitimate purpose, such as criticism and accreditation (al-jarh wa al-ta'dil) and clarifying the state of someone who openly commits فسق (transgression/immorality). If a person openly commits فسق, it is permissible to backbite about him for the purpose of warning against him. However, it is not permissible to backbite about him to disparage him, under the pretext that he thinks his action is good. A Muslim must monitor himself when speaking about others, for even apparent reasons like religious zeal and anger for the sake of Allah can be entry points for Satan; such as marveling at evil, mercy, and anger for the sake of Allah, which may lead to backbiting unintentionally. These three matters are subtle, and Satan makes a person backbite while he thinks he is doing good. The correct approach is that it is not permissible to mention the person's name in these cases unless there is an essential need to mention the name, such as clarifying the state of someone who openly commits فسق.