Saito acts as the catalyst for the entire movie. He's the one with the agenda in the film. He's not the man with the plan… but he's the man who makes the men plan.
Without Saito's goal of breaking up the energy monopoly of Fischer Morrow, there would be no need of inception and hence no movie. The question is: can we trust Saito? Cobb and the gang are just happy to have a job, but as viewers we wonder whether Saito is taking advantage of Cobb's desperate need to get back to his family.
Is he really on the look out to stop runaway capitalism or is he being a manipulative businessman? We never find out, but seeing as he is a man of his word and pulls some strings so that Cobb can get back to his children, we'd like to assume the best. Emphasis on the "like to."
Poor Old Saito
Things take quite the turn for Saito in the dream world once he's shot. It turns out Eames was right: the job was too complicated for a tourist tag along, but inception is a tricky business and Saito would have no way of knowing whether it had occurred unless he was there.
He actually never did see the final product but he assumes that Cobb succeeded anyway…maybe he is just a great guy.
Let's rewind for a moment to the point where Cobb is leaving the helicopter, about to reject Saito's offer. Saito asks him if he wants to see his kids or if he'd rather be an "old man filled with regret, waiting to die alone."
Ironically, that is exactly what happens to Saito. He may have only been in Limbo a few dream-Level- #3-minutes ahead of Cobb (which is mere nanoseconds in the real world) but that was enough to make it seem like an eternity. When Cobb meets him he is very old and very confused. He appears to have forgotten not only the real world but also himself, and his projections have more life than he does.
Then Cobb jogs his memory and he repeats what he once said to Cobb—except now he's referring to himself. He has been waiting to die and Cobb has come to kill him. At the beginning of the movie this seems morbid but at the end we know that this is his return to reality. He can die and come back to reality (and he can finally get off the longest flight in the history of forever).