Is it permissible to respond in kind to those who slandered, insulted, caused reputational damage, and spread lies, and is there any sin in doing so?

Patience with the harm inflicted by people is a sign of strong faith. Islamic texts encourage forgiveness and suppressing anger. There is no objection to seeking assistance from good people to repel injustice or to refer the matter to the judiciary, provided that this does not lead to a greater harm. As for retaliating for adultery in kind, it is impermissible, because it is an act forbidden by Islamic law. The Law has prescribed a punishment for the adulterer and has not permitted retaliation in kind. Repentance from adultery must be immediate. Similarly, those who have used magic must repent, for it is one of the gravest sins.