Is it permissible to apply the term "the transgressor" to Mu'awiyah, may Allah be pleased with him, and which scholars have used this term for him, and is it permissible to say that his error was not an error of ijtihad?

The conflicts that occurred among the Companions, whether through fighting or abstaining from it, were based on their independent judgment (ijtihad (definition)). It is necessary to refrain from delving into their disputes, and instead, to love them and praise them. Al-Nawawi said: "The blood that flowed between them is not included in the warning (of punishment). The doctrine of the Sunnis is to think well of them and to refrain from discussing their disputes. Their fighting is interpreted as them being mujtahid (exercising independent judgment) and mu'awwil (interpreting matters), not intending disobedience. Some were correct and some were mistaken but excused." Refraining from discussing the disputes among the Companions and focusing on their virtues is the path to safety. Naming Mu'awiyah's army as 'rebels' (bugaah) does not necessitate ruling them as sinful, for it was a matter of independent judgment. Ibn Hazm explained that Ali was correct and Mu'awiyah was mistaken in prioritizing seeking retribution (qawd) over pledging allegiance (bay'ah), but he was rewarded for his independent judgment. The mujtahid who errs bears no sin and receives a reward. Allah Almighty referred to the two warring factions as believers and brothers, which indicates that their fighting did not expel them from faith.