Misaki doesnt understand Scepter 4 until then. This was the issue to begin with Scepter 4, the Blues, arent Misakis style at all, and we dont even need to go into reasons for that its clear. But now that Misaki sees that Saru isnt a bad guy just because hes in the Blue-coats now, he starts to try to understand them. Theres the part in the Christmas Eve battle, when he and HOMRA are waiting to fight Jungle, when he starts going off about how Munakata doesnt seem like a good leader because of how hes not in the parts of the building where the fight is. He understands the strategy, but he says its dishonorable, and he wonders why anyone would want to follow him and then hes surprised when Kusanagi points out that Seri is there. Misaki gets flustered and says, Oh, no, I didnt mean you he just meant Saruhiko. Also note his genuine confusion when Saruhiko goes off, before his (staged) fight with Reisi.
So how is Misaki finally able to realize Saruhikos feelings? Reisi tells him, plain and simple. And its because he can now see Saruhikos loyalty that Misaki can now forgive him. It's because he can then see Scepter 4 as they are - what they do, why they do it, why they're so uptight, etc. It's because they have a mission. That's when Misaki realizes that. When Misaki tells Saruhiko that hes not a traitor, and thats hes actually really cool, thats when Saruhiko can understand and accept it for himself.
And what makes him really cool to Misaki at that point is his loyalty. His loyalty to his Clan and his King, even if theyre not Misakis. He understands that Saru would do anything for Scepter 4, just like he would for HOMRA, and thats what gets Saruhiko his approval. And when Misaki says it, thats when Saruhiko realizes that about himself.
(mostly from a blog post on this...)
So is he a traitor? No, as Misaki says - the Blue King was his king all along.