What is the ruling on invoking Allah's mercy upon Yazid ibn Mu'awiya, knowing what he did to the people of Madinah?

Yazid ibn Mu'awiyah is from among the People of the Qibla (Muslims). The verified position is that it is not permissible to curse him or declare him a disbeliever, because he was a sinful Muslim, and his sinfulness does not expel him from the fold of Islam. A sinful believer is not declared a disbeliever due to his sin unless he considers it permissible. Similarly, it is not permissible to curse a specific sinful individual, because cursing is expulsion from the mercy of Allah, and one does not know a person's ultimate fate unless he dies upon that which warrants cursing him. However, it is not appropriate to show him mercy or pray for his forgiveness, because he committed grave acts.