Re: The Joker's background?
Dont read too much into it. The Joker lies to scare people. The only thing worth noting is the story he told to Rachel. If you take a closer look you'll see how it applies to Rachel and Bruce.
Wife gets carved = Rachel fights crime as a DA
The joker tries to sympathize with his wife but takes it too far = Bruce fights crime but takes it too far.
In both cases the women dont appreciate the men of the story.
Of course the joker didnt know all that so for him it was just a random story he decided to tell. But if you examine it, you'll see how Nolan is telling us how Bruce must feel.
And before anyone draws conclusions out of this, of course the story is fake and there is no real parallel between Bruce and the Joker and the women in their lives.