Re: Why did the League of Shadows want to destroy a peaceful city?
There is mass social and political corruption. Street criminals are behind bars but men like Daggett are still manipulating those under them and aren't opposed to dealing with shady characters like Bane to get what they want.
Also, the League of Shadows is an extreme fundamentalist group. Since when do fundamentalists ever make sense? Their doctrines are always extreme and questionable. Bottomline: They still see Gotham as rotten to the core and it needs to be cleansed. Bane probably felt especially vindicated when he discovered Commissioner Gordon's speech about Dent.