Re: Why did the League of Shadows want to destroy a peaceful city?
It was thematically important for the LOS to return for several reasons.
1. They didn't believe Gotham could be saved. Even if there is peace in Gotham, it's just a bandage. When they say Gotham is "beyond" saving, they mean it.
2. Legacy is a major theme of TDKR. Ra's al Ghul teaches Bruce how to create a legend in BB. In TDKR, he teaches Bruce how legends can achieve immortality. Ra's lives on through Talia and Bane, just as Batman will live on through the people of Gotham and Blake.