What is the ruling on insulting and mocking an oppressive man, especially if he mocks Islam?

Whoever openly commits injustice and wickedness, it is permissible to revile him (sabbuhu), backbite him (gheebatuhu), and supplicate against him. Among the proofs for this is that an open transgressor has no sanctity, and it is permissible to mention him with what he openly commits of wickedness. He who openly displays evil has no immunity from backbiting (la gheebata lahu). However, mocking and ridiculing scholars or callers to Islam because of their religion and uprightness is disbelief (kufr), because ridiculing believers is ridiculing Allah, His verses, and His Messenger. Nevertheless, it is not permissible to backbite and revile someone who openly displays evil if that leads to a greater harm.