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According to Clive Owen it's deliberately ambiguous
Blackfilm.com
What builds your character, what makes him tick? Is there going to be a sequel to this movie because you walk out with the document, the Nazi document, is there going to be a sequel?
Clive Owen: Not that I know of. What drove me was the idea of playing this guy who pulls off this very extraordinary bank robbery and has his own reasons for doing so. It looks like it's one thing, he takes hostages and it looks like it's going to be a very violent sort of, this guy in there to rob the bank and make a lot of money for himself, and that isn't quite the case. The guy's motivated by other things and it is ambiguous, it's not a straight forward, clear cut thing. I think that goes for every character in the movie. I think that every character is very rich and ambiguous. Also I think that Spike added, it wasn't in the script, it's full of that New York flavor. You've got lots of great character actors in from New York and there were lots of scenes that were improvised and just he really fleshed it out and made a very smart script into something even richer I think.
Dalton (Owens) is the "Everyman" in this movie. What the world sees, what you and I see, is that in most facets of life the rich get richer, the poor get poorer, and rarely does any high-ranking white collar criminal ever pay for their crimes. Think about the housing bubble of 2008. When that bubble popped, nearly every large bank and Wall Street firm was culpable, yet not a single one of the people responsible spent a minute in jail. People went bankrupt, but no one paid for it.
This is just one example of a larger issue; the frustration of the poor to middle class about the fact that the upper class rarely has to atone for what they do. Yes, there are the occasional people like Bernie Maddoff and Kenneth Lay who pay the price, but usually only after other members of the upper class themselves get ripped off.
Dalton is our hero. He's the guy who can make the rich people pay for their mistakes. His point was, I'm good enough to outsmart the untouchable upper class. I can beat them in the ultimate game of Chess. I can make them pay for their crimes. And, frankly, it would have made a great franchise if they took it further. Sort of like Oceans 11, but with a Robin Hood feel.