Shariah Ruling on addressing Yazid with "Radhiallahu Anhu"
The words "Radhiallahu Anhu" meaning may Allah be well pleased with him are used to signify Allah’s pleasure. They are used as a form of respect to praise somebody. "Radhiallahu Anhu" is used especially for the Companion and for those pure souls, whose hearts are filled with fear and awe of Allah, as Allah Most High himself says: Surah Bayyinah-08.
The aforementioned Hadith and the interpretations of the Imams make it clear that Yazid was wretched. He changed the Sunnah, he tried to destroy the religion, he insulted the 2 blessed sanctuaries and he insulted the Prophet’s Family. Using "Radhiallahu Anhu" for such a person is to respect him and this is equal to destroying the edifice of Islam. This invites Allah’s anger and misguidance. The Holy Prophet (Sallallahu alaihi wa sallam) has declared respecting an innovator equal to lending a hand in destroying the edifice of Islam.
There is a Hadith in Imam Tabarani’s Mo’jam Ausat (Hadith No: 6263):
Translation: It is narrated on the authority of Hadhrat ‘Aisha Siddiqua (May Allah be well pleased with her) that the Holy Prophet (Sallallahu alaihi wa sallam) said: Whoever respects an innovator, verily he helps in destroying Islam.
There is a Hadith in Imam Baihaqi’s Shu'abul Iman:
Translation: It is narrated on the authority of Hadhrat Anas (May Allah be well pleased with him) that the Holy Prophet (Sallallahu alaihi wa sallam) said: When a innovator is respected, Allah expresses His anger and because of it, the ‘Arsh trembles. (Shu‘ab Ul Iman, Hadith No: 4692)