Superhero origins all pivot on circumstance. No self-destructing Krypton, no Superman on the Kent family farm. No radioactive arachnid, no Spider-Man. No misused gamma rays, no Hulk. And Batman's origin hinges on circumstance as well: no murder, no Batman. But Batman’s origin is not as simple as that.
What is noble about Batman/Bruce Wayne is that he chooses his path; he is a product of free will and determination, not chance. His parents may have been murdered, but he isn’t the only mourning son of slain parents. Unlike Superman, Spidey, or Hulk, the circumstance that grips Wayne’s world is shared by others. What differentiates the Dark Knight is that he embarks on his journey consciously. Spider-Man’s “great power” is thrust upon him by fate, forcing him to accept “great responsibility”. Wayne chooses the burden of great responsibility that sets him about a journey for establishing himself as a great power. Batman is a Horatio Alger-character – self-made and reliant, his effectiveness hinging upon cunning and ingenuity, not an inexhaustible supply of inherited power – while the others are more members of the Lucky Sperm Club. (This is particularly ironic because of massive wealth of the Wayne family.)
Very few other superheroes - and none of the big guns - choose their path. Clark Kent, Peter Parker, Bruce Banner, and Logan all have no choice in their alter-egos. They are victims of fate, subservient to the surrounding world. And my guess is that if Clark, Peter, Bruce and Logan were asked whether they would give up their superpowers to become regular humans, each would do it in a moment for a chance at normalcy. Wayne had normalcy and willingly embarked on a different path. His genesis was conscious, not manipulated by the heavy-hand of fate.
While Kent and crew painfully embrace their alter-egos, Bruce Wayne is the only one who truly becomes his alter-ego without reservations and emotional hindrance. And in doing so, Batman becomes the man's true ego, while Bruce Wayne becomes his alter ego. I am wont to believe that any character or person who chooses to become "something else entirely" - as Ducard says at the beginning of "Batman Begins" - assumes that "new" identity as their ego, relegating their previous life to their alter-ego. That is why I was so thrilled with how "Batman Begins" ended, with Rachael Dawes acknowledging that, "your real face is the one that criminals now fear. The man I loved - the man who vanished - he never came back at all. But maybe he's still out there, somewhere. Maybe someday, when Gotham no longer needs Batman, I'll see him again." That is EXACTLY right. He has become something else entirely and in doing so, marginalized another part of his life. By choice, not by chance. And this is what differentiates himself in the crowded superhero canon. Since Wayne chooses to assume the Batman identity, Bruce Wayne becomes the alter-ego, not vice versa.
(A quick aside and perhaps a point of clarification - based on this standard, James Gatz is the alter ego to Jay Gatsby because Fitzgerald's protagonist willingly chooses to be Gatsby rather than Gatz. Although this is slightly complicated by the fact that Gatsby is never bound by dual identities like Batman/Bruce Wayne is.)
And Batman is clearly a man on a mission, but it's not pursuit of personal vengeance. His aim is much higher than that. He wants Gotham to be a better place, a city where a young Bruce Wayne would not become a victim. In a way, he's out to make himself obsolete (this will be a central thematic tension in the new film). Spider-man, Superman, and Batman are all heroes who wish they didn’t exist. The difference is that Spider-Man and Superman wish that fate had not dealt them these super cards, while Batman wishes he lived in a world where he was not needed.
The cumulative effect of this is that the world Batman inhabits – the world without flying men and purple-pants wearing angry monsters, but with crime, grit and corruption – is that much more believable to us, the audience. We can understand how a man became a cape-and-cowl-clad creature of the (k)night and in some small way relate to him in a way that others can never be relatable to those watching their exploits. We can understand and empathize with Peter Parker, but not with Spider-man. We can feel for Clark Kent, but not for Superman. But we can emphasize with both Batman and Bruce Wayne. Now as for the Joker…