I know he continued fighting for a while after he knew his army killed Amar ibn Yasser, saying something like "the army that brought him is the one who killed him" which was wrong of him, he should have stopped immediately after he knew.
The thing is, him not stopping and then reinterpreting the hadith tells about how strongly he felt his position of not going forward until all traitors were caught and killed was right, which strongly indicates he only started this war when he thought he was 100% in the right.
Am I being generous with excuse? Of course I am, he's muawiya ibn abi sufyan, one of the writers of the Quran, a just and generous king, and the best king the muslims saw after the four caliphates.