When does a Muslim forgive an aggressor, and when does he not, especially if the aggressor is a person of power and influence?

The basic principle is that forgiveness is recommended and beloved in Islamic law; it is a path to honor and to attaining Allah's forgiveness. It is permissible to forgive an oppressor, unless it leads to corruption, such as the oppressor persisting in his injustice, or others being emboldened to commit injustice. Justice may be better than forgiveness if forgiveness leads to an increase in oppression, or if retribution serves as a lesson for others. Forgiveness is conditioned upon reform; if the Shariah interest dictates punishing the offender, then forgiveness is not commanded.