So, is Saul the mole or not?
In Season 1, Saul Berenson is so upset about what is going on with his wife at home that he initially fails simple questions during a lie detector test - questions like "is your name Saul Berenson?" Nicholas Brody later (in the same episode) shows us that it is possible to beat a lie detector test when he claims he never cheated on his wife, when we know he just spent a weekend screwing Carrie's brains out. Around the same time, someone slips a detainee they were holding a razor blade, which the detainee promptly uses to commit suicide. At the time, Nicholas Brody was suspected. This mystery was left unresolved. To further complicate things, Saul appears to recite a few words of Arabic in seeming solidarity or with sympathy toward the dead detainee - turns out this was actually a Hebrew prayer (Saul is Jewish). Of course, we later find out Brody is far from innocent, but his character doesn't take action toward supporting any terrorist efforts until later in the season. We also know the dead detainee was Brody's interrogator - a man Brody hated for mercilessly torturing him and who he had little incentive to help.
By the end of Season 1, Saul is using information regarding a secret drone assault against the infamous Abu Nazir's compound to blackmail the Vice President. Throughout both Season 1 and 2 of Homeland, Saul Berenson has his hands in nearly everything going on. If he's out of the loop, he's doing whatever he can do to get inside (see his extensive efforts to figure out why Quinn was put in charge after they began using Brody as an asset against Abu Nazir). He does a lot but doesn't seem to make any real contributions - his most effective role is as Carrie's caretaker and mentor.
One thing Saul does manage to do is discover the computer chip containing Brody's suicide video. He goes first to Carrie and then Estes individually with the video and appears to try and shape their reactions to it. Saul steers Estes away from revealing what Brody might've done to the Vice President because Estes's failure to sense the plot might doom his long-term efforts to become CIA director. The thing is, why would Saul care about Estes? They're hardly friends. Saul convinces Carrie not to confront Brody and Estes not to arrest him so that they can Brody to find Abu Nazir. Or, if we use our 'Saul is the mole' logic, Saul does so because he knows Brody is integral to the last leg of Abu Nazir's terrorist attack, the results of which will put him in charge going forward.
Once Saul discovers that Estes intends to assassinate Brody after he has helped them capture Abu Nazir, Saul goes ballistic, quickly imperiling his own career and very nearly leading to his dismissal from the CIA. Quinn, Brody's would-be assassin, later decides on his own not to kill Brody, much to Estes's dismay. Saul's reaction to the planned assassination of Brody seems out of proportion. We know from his interactions with Carrie that Saul thinks very little of Brody - he's scum, a terrorist collaborator, a jihadist. At the end of Season 2, just before much of the CIA's leadership is assassinated in a car bomb attack, Saul asks Carrie to accompany him at sea to dump Al Nazir's body. Saul consistently advises Carrie against any further contact with Brody even after Brody probably saved her life and helped them nab the most famous terrorist in the world. Also, just before the attack, when Saul informs Carrie of an impending promotion, he is incensed when Carrie indicates she might leave intelligence work altogether.
The 'Saul is the mole' line of logic makes sense. Throughout Seasons 1 and 2, it's clear Abu Nazir is getting information from someone inside the government's intelligence apparatus. Saul is pissed about drones for the same reasons Brody is pissed about drones. He wants to keep Brody alive because Al Nazir needs Brody to drive his car to the Vice President's memorial service in order to complete the terrorist attack - an attack which just so happens to leave Saul Berenson in charge. If Quinn assassinates Brody, the attack is ruined, hence why Saul makes such an effort to thwart Estes's assassination plans. Saul wants to keep Carrie away from Brody because he knows another attack is coming.
But 'Saul is the mole' is certainly not the only explanation for Saul's actions. Most shows have some character that the audience is supposed to look to in order to decide what's right. This character has some principle or set of principles that the audience can trust he or she won't violate. Saul Berenson could be that character, which would explain why he is upset about secret drone attacks, Carrie's relationship with an admitted would-be terrorist, the secret assassination of a US Congressman, and so forth. Maybe his expressions of sympathy toward the dead detainee only indicate Saul's humanity - his reaction, after all, was the same among the CIA dead at the end of Season 2. Maybe no one slipped that detainee a razor blade - maybe his handlers just missed it, which was one of the hypotheses initially put forward during the episode. Maybe the show's creators are showing us how the CIA's ways of thinking can lead to false positives or false conclusions. The CIA's paranoia leads them to suspect one another.
Before his death, Estes appeared happy to frame Saul for being the mole for his interference in their planned assassination of Brody. If you'll remember, the primary evidence against Saul involved his alleged assistance in the suicide of another detainee, a young woman he bonded with on a road trip from Mexico to DC. Their reasoning for suspecting him of course would involve the first detainee's suicide, also when Saul was nearby.
So is Saul the mole? It is certainly possible that Saul may not be a mole at all - maybe it's someone else yet to be revealed, or someone among the dead in the aftermath of Abu Nazir's final attack. I have a feeling that, come September, this question won't be quickly resolved.