Is Ibn Taymiyyah's description of Ibn al-Mutahhar al-Hilli as "Ibn al-Munajjis" (son of the defiler) accurate, and is this act considered permissible and lawful as a form of insult, and what is the wisdom behind this description?

Ibn al-Mutahhar al-Hilli is Jamal al-Din al-Hasan ibn Yusuf ibn Ali ibn al-Mutahhar al-Hilli (648-726 AH). He was one of the foremost theologians of the Imami Rafidah and the Mu'tazilah. He authored a book on the Imamate and the doctrines of the Rafidah, which he titled "Minhaj al-Karama" (The Path of Nobility). Shaykh al-Islam Ibn Taymiyyah refuted it with his book "Minhaj al-Sunnah al-Nabawiyyah" (The Path of the Prophetic Tradition).

It has not been found in Ibn Taymiyyah's books that he described Ibn al-Mutahhar as "Ibn al-Munajjas" (son of the defiled/impure one). However, he did describe his book "Minhaj al-Karama" as "Minhaj al-Nadamah" (The Path of Regret), and he described it as being more deserving of impurity and defilement than purity because its heart contains malice towards the best of believers. He likened him to the Jews in their wickedness and pursuit of whims, and to the Christians in their extremism and ignorance. Shaykh al-Islam intended by this to repel people from Ibn al-Mutahhar and to demonstrate that his belief is based on impurity and slandering the best of believers, and that his book on the Imamate is a book of regret, not nobility.