Re: Batman Begins v.s The Dark Knight Rises
I can see where Talia was coming from. She hated Ra's for throwing Bane out of the League. Then he's gone, and she can't reconcile with him. Before Bane found Gordon's confession, he had been recruiting Gothamites that were put out of work. Talia had been in Wayne Enterprises for an unspecified number of years, and would have seen corporate greed as the source of job losses in Gotham. That would allow her to line her goals up with the League's purpose. I believe that Talia fixated on Gotham because it was the city where her father was killed. So completing that mission for him would make sense, in terms of honoring him.
"I never asked for this crown. Gold is cold and heavy on the head, but so long as I am the king, I have a duty … If I must sacrifice one child to the flames to save a million from the dark … Sacrifice … is never easy, Davos. Or it is no true sacrifice." - Stannis Baratheon