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 am for no master nor moral persuasion. I am for myself. What your yearning soul, madam, might mistake for loyalty to person or Purpose is merely a firm and, aye, principled determination to accept responsibility only for myself and my own actions." - Elric of Melnibone, The Revenge of the Rose (Elric: Swords and Roses).