I just caught the ending of this episode (cause my TV messed up and I didn't catch what they were saying), and his behavior did seem a little suspect. The fact that Tenzin mention that only the Avatar could spiritbend makes me wonder if he has some possible connection to Amon. Old acquaintances turned enemies? Who knows.
In his story (if it's true), he mentioned how his family were farmers, and that he was scarred by a firebender, hence why he wears a mask. Taking that into account, I've read The Promise comic, and part of that story deals with the Harmony Restoration Movement moving, even forcing, certain settlements back to their country of origin. However, some of the tribes were reluctant and even opposed to this, as shown with Zuko's confrontation with that one family.
What if his family was part of one of those settlements that resisted the Harmony Restoration Movement, the main purpose of which was supposed to bring balance to back to the world? It would certainly fit in with Amon's sentiment that bending doesn't bring balance and how it's been responsible for many wars throughout history.
Also, when he mentioned that he was scarred by firebender, it made me think that Zuko was somehow related to that incident in some way. I'm not saying it was Zuko himself, but maybe his actions sort of have an indirect effect on what happens to Amon. That part is probably a bit of a stretch, but it makes you wonder.
On top of that, I don't it's a coincidence that both Mako and Amon mentioned that both of their parents were attacked/killed by firebenders? Maybe there was some past event (the Harmony Restoration Movement) that caused this?