He is shown to be an extremely smart person who planned and actually executed an excellent bank robbery.
In the first scene of the movie he is shown breaking the fourth wall where he says:
Recently I planned and set in motion events to execute the perfect bank robbery.
That's also the "when."
As for the "why," beyond the obvious financial motivation, it's exceedingly simple.
Because I can.
This hints towards one of his motivations and the confidence in doing so.
In the later part of the film he tells Madeleine (the fixer hired by Arthur Case, the chairman of the board of directors):
Let me tell you a story. During World War II, there was an American working for a bank in Switzerland. Now, I don't need to tell you that this period in history was rife with opportunity for people of low morals. People like this man (Arthur Case). He used his position with the Nazis to enrich himself while all around him people were being stripped of everything they owned. Then he used his blood money to start a bank. Now, does this sound like it might be the man you work for? Or am I just whistling Dixie out of my ass?
This essentially portrays that Dalton is stealing from just another bad guy. There are other incidents as well where Dalton's possibly good side is shown.
- He doesn't actually shoot a victim but just fakes it.
- In the face-to-face conversation with Detective Keith Dalton tells him that money shouldn't matter if he really loves his girlfriend. (This conversation was bit funny.)
- In the end of the movie Keith finds a diamond in his pocket which was left there by Dalton (as a gift) when he bumped into Keith while leaving the bank.
- His conversation with Brian, an 8-year-old boy who is playing a violent video game on his PlayStation Portable.
And finally in the end when Dalton is leaving the bank there is a short monologue by him which tells quite a bit about his character. That he doesn't want to kill people but just steal.
What was Dalton's point? Did his character have any depth beyond what was portrayed? Or he was just another smart villain stealing from other bad guys?