Well, when a Circle that was literally teaming with abominations and demons doesn't get Anulled, it might make it seem to him that no Circle or mage is beyond redemption. =)
That, and he basically takes back what he says about mages in Act I, but no one ever seems to want to forgive a recently transferred PTSD Templar from being a bit... nervous about mages and their abilities. For what he went through, having that knee jerk reaction about mages is perfectly acceptable. Especially when he mellows a bit afterwards. He actually mellows far more than Fenris does and people still hate. Sigh.
Again perhaps, that could be his mindset on it, or perhaps he simply views it as a waste of a potent resource, eitherway he wasn't for the motion to destroy the circle due to Anders i grant that, but i do believe people are overestimating what that implies personally.
Given that.
Except that when PC interaction is granted, Templars have a duty to combat and destroy Blood Mages, There are lines that the Templars are not meant to cross, and sparing them is one of them. Apostates, Circle Mages are one thing, But those who consort with Demons? It crosses a line.
Point is, He was still a Templar, He acting as one, my point is that just because he was against Meredith by the end game doesn't magically make him Pro Mage.