After his murder...
After his murder, fitnah arose and a viewpoint developed that those who fought against Hadhrath 'Ali were fasiq because they rebelled against the rightful Imam".
One who turns his back on the right path is a Zaalim and Fasiq In Sharh al Maqasid Volume 2 page 306 Allamah Taftazani states that: "The battles between the Sahaba are proof that some companions left the right path and became Zaalim and Fasiq because they became affected by jealousy, hatred, hypocrisy, a desire for power and indulgence because not all the companions were just, not was every individual who saw Rasulullah (s), good".
Sharh al Maqasid Volume 2 page 306 Clearly the right path was to attach themselves to 'Ali (as). By turning their backs on him and fighting him, proves that Mu'awiya and his party had gone astray. He fought 'Ali and caused the death of prominent companions. Thereafter as a Khalifa he adopted a policy of oppression against the lovers of Hadhrath 'Ali and cursed him during the Friday Sermons. Do we need any further proof to demonstrate that Mu'awiya had gone astray and hated Imam 'Ali (as)?
Mu'awiya continued to act in the way of descendants. His brother, maternal grandfather and uncle were killed at the hands of Maula 'Ali (as) at Badr. Mu'awiya was hence motivated by hatred and dedicated his life to fighting Imam 'Ali (as). Mu'awiya's Conquests Some advocates of Mu'awiya commonly highlight the fact that the Muslim Empire was expanded under his rule with Sahaba under his helm, as was not the case under Imam Ali (as)'s khilafath.
It should be pointed out that empire expansion means little in the eyes of Allah (swt). One will be questioned on the day of judgement on his 'personal deeds' and Mu'awiya despite his advocate's poor defences, will indeed have a great deal to answer for.
In any case, his conquests mean nothing, as Imam of Ahl'ul Sunnah 'Abu Bakr al Jassas states in Ahkaam ul Qur'an Volume 3 page 119: "Following the four khalifas the Sahaba participated in Jihad under the helm of Fasiq's and Faajirs, 'Abu Ayub Ansari participated in Jihad under the Leadership of Yazeed".
Mu'awiya's Government compared by Hadhrath Ayesha to Firawn (Pharoah) and other Kaafirs (Pagans) Shaykh ul Mudira page 165 al Bidaya 131 Volume 8 Mukhtasar Ta'reekh al-Dimishk Volume 25 page 42 Aswat bin Yazeed said to Hadhrath Ayesha: 'Aren't you surprised that this Mu'awiya is from Tulaqa (freed captive) and in Khilafath he fought the companions?