What is the position of religious scholars on Mu'awiyah's declaration of succession for his son Yazid, and is that considered a violation of the principle of Shura, and what is the ruling on Yazid, who killed Imam Hussein, may Allah be pleased with him?

Mu'awiyah, may Allah be pleased with him, was among the best kings of the Muslims, despite his error in appointing Yazid as Caliph. He had commendable efforts and blessed deeds in Islam. Umar, may Allah be pleased with him, appointed him over Syria, and people loved him for his justice and forbearance. The Prophet's supplication for him has been authentically reported. It is not permissible to disregard his virtue for errors that were a result of ijtihad (independent reasoning) or minor mistakes when compared to his immense good deeds. This aligns with the methodology of Ahl al-Sunnah wal-Jama'ah in dealing with great religious figures who may commit errors.