Is it permissible to accuse Muʿāwiyah ibn Abī Sufyān of usurping the Caliphate from ʿAlī, may Allah honor his countenance, and of rebellion?

We must think well of the Companions and interpret their actions in the best possible light. Therefore, it is not permissible to accuse Mu'awiyah of usurping the caliphate. He believed in the necessity of punishing those who wronged Uthman and was not ambitious for the caliphate. Rather, he considered Ali to be superior and more deserving of the matter than himself. Many scholars have concluded that Ali was in the right, and that Mu'awiyah was a mujtahid (one who strives to form an independent judgment) who erred in his ijtihad, and he will be rewarded for his effort.