What is the ruling on one who says: "I bear witness that Allah has led this human astray knowingly"?

It is permissible to describe the people of misguidance with evil if the speaker is among the people of knowledge and piety, with the aim of warning the Ummah against their misguidance, and this is part of the required advice and clarification. However, if the speaker is unqualified or if his intention is to mock and defame, then it is not permissible. Imam al-Nawawi permitted backbiting for a beneficial purpose in six situations, among them: warning Muslims from evil, such as discrediting narrators and witnesses, and when there is fear for a student of Islamic jurisprudence that they might learn from an innovator or a transgressor, provided the intention is sincere advice and not envy or the whispers of Satan.