Wow, Great find! Sorry but as usual the Shia miss the bigger picture like you have done. Yes, that is what Ali may have thought about the other side's leader, likewise the other side thought something similar about Ali for not implementing justice in the case of Uthman. It was a Fitna between the believers.
More complex? Well, read this, Ali bin abi Talib himself answers you:
"The whole thing began thus that we and the Syrians met in an encounter although we believe in one and the same Allah and the same Prophet, and our message in Islam is the same. We did not want them to add anything in the belief in Allah or in acknowledging His Messenger (Allah bless him and his descendants) nor did they want us to add any such thing. In fact, there was complete unity except that we differed on the question of `Uthman's blood while we were free of responsibility for it." Letter 58
We don't miss the bigger picture, you disregard the bigger picture by putting forward bits and pieces that suit you and ignore the others.
The whole thing boils down to one thing, Ali was the rightly guided Caliph of the Muslims and the Ulul Amre of the time. Muawiyah disagreed with Ali over Usman's blood, in other words Muawiya wanted to dictate Ali in how to deal with the murder of Usman.
He put his demands forward and threatened the Islamic Republic, the Islamic Caliphate, the Islamic government that if his demands weren't met over the killing of Usman then he will use his influence and support and will turn towards violence which he eventually did.
According to the Ahle Sunnah Wal Jama'ah Muawiya was on the wrong and Ali was on the right and on Haq. You handful of people from a minor group claiming to be Ahle Sunnah can jump up and down all you want. You can't mess with reality and facts.
Ali was the rightly guided Caliph of the Muslims and the Ulul Amre of the time. Who ever opposed for what ever reason is batil.