Good Will Riker wrote:
I will have to go back and rewatch Casino Royale on DVD to confirm this.
But, Le Chiffre's "Your friend Mathis is my friend Mathis" line as well as the suspicious powder inside the cup Mathis offers to James Bond is quite telling, eventhough M says otherwise at the end.
CR keeps it ambiguous whether or not Mathis was a baddie or not. At the end of the movie, the conversation between Bond and M goes along these lines:
M: At least this clears Mathis.
Bond: It just proves that she's guilty; not that he's innocent. It could have been a double blind.
They leave Mathis's intentions open, although several things in the movie suggest he really was with the baddies (although the alka seltzer he gave to Bond never really made me think he was giving it to him for nefarious purposes - that wouldn't make sense to kill Bond with the bad guys still needed the damn passcode [plus Vesper's deal]).
QoS shows that Mathis was innocent all along clearing up the ambiguity from CR.