People whom one is allowed to Backbite

The Prophet (SAWA) said, ‘Four types of people are such that talking about them behind their backs is not considered backbiting: the immoral person who makes a public display of his immorality; the dishonest leader who, even if you were to be good to would not appreciate you and if you were to be bad to would not forgive you; those who joke about their [and others] mothers; and one who is a dissenter, who defames my community, and draws his sword against it.

The Prophet (SAWA) said, ‘Until when will you sidestep talking about the impudent person?! Disgrace him [through disclosing his insolent acts] so that people may be on their guard against him.