Khawarij were a quite deviant, dangerous, and misguided group who did the greatest evils against Imam Ali(PBUH). However, Imam Reza(PBUH) describes Moawia and his followers as more deviant than Khawarij so much that if there were resources enough to fight one of them, fighting Moawia would be the priority.
Ibn Abi al - Hadid in his “Sharhe Nahjal Balagha”‌ elaborates this issue as follows: “many of our followers doubted the religious beliefs of Moawia and did not stop calling him profligate but also believed him to be infidel. They narrate him to have said some words and sentences proving his disbelief; like Zobei Bin Benqar, a non - Shiite figure(this is proved by the fact that he disliked Imam Ali(PBUH) and rejected him), who accuses Moawia of disbelief. He says “the son of Moqaira Bin Shaaba recalls ‘I went to Moawia with my father who went and spoke to him often and on his way back he admired his outstanding wisdom and talent. But one day he came back after meeting Moawia in such a sad mood that he didn’t even eat dinner! I waited for some minutes but he was so sad that it seemed something was wrong. Why are you so upset? I asked. I am coming from the most evil and infidel person, he answered. I asked the reason and he said I was talking to Moawia in private and told him you are getting old and it would be proper to act just and engage in good deeds. You are getting old and it would be better if you do good to your brothers in Bani Hashem and visit them because today they are no danger to you and history will remember your name for good”‌. “Moawia got angry”‌ and said: “what name to remember? Abu - Bakr was vicegerent and acted justly and did important things for many years, but he was forgotten after he died and people only say “Abu - Bakr was so and so!”‌ These words made Moqaira Bin Shaaba to call Moawia as the most infidel person!Translated by Sadroddin Musawi