Jack Napier was a soulless entity concerned only with his outward appearance and vanity. When that was taken from him, he lost the only thing he cared about - himself - in a bizarre parody of tragedy. This is why he became the "world's first fully functioning homicidal artist." Desecrating all that was good to bring it all down to his level. Joker became a walking incarnation of tragedy, dancing throughout every scene (he dances while killing Grissom, dances when he finds Vicki's picture, dances out of her apartment after seemingly killing Bruce, dances at the top of the clock tower... all are "dances with the devil"). Its no coincidence that the Joker's demise is brought about by a gargoyle, medieval statues placed on buildings to ward off demons and devils.
Bruce, on the other hand, is devoted to stopping the tragedy that befell him from ever occurring again. He is the walking antithesis to tragedy, a shadowy force of justice. The fact that the Joker killed his parents, which is THE defining tragedy for Bruce, only emphasizes this antithesis even more. He's a bit like a living gargoyle, a winged terror meant to combat evil. "Its not a perfect world," he says, because tragedy is real and it exists... but he's out there to fight it to the death all the same.
Vicki is for both characters a sort of mediating force. She's connected to the notion of a normal life. In the darkness of Gotham, she's a bright blonde. She's beautiful. She's charming. She's in some ways an angelic, pure figure. But she's more than a pretty face or an image of good - she's a mediating force because she's acquainted with tragedy - she's photographed war-zones and death. She's a bridge between that tragedy and the idea of a good life. At the same time, however, she doesn't quite understand Bruce and can never fully understand Bruce. She's acquainted with tragedy, but only from behind a camera lens: she's a step removed from it, a step outside it, and while she's fascinated by it (and by Batman) she doesn't really "know" it or understand it.
