Re: Why did the League of Shadows want to destroy a peaceful city?
Talia wanted to complete her father's intentions to destroy Gotham. Killing Bruce was just a reward for not obsessing over it. She had no plans to kill Bruce before she had finished with Gotham. If that was the plan than trying to get Bruce to leave with Talia or not have Bane kill him instead of putting him in the pit would have occurred. The siege of Gotham served two purposes: 1) Too show the world savagery of people, and 2) show to Bruce that everything he had done as Batman was in vain and that the people didn't believe as he did that Gotham could still be saved. Bruce returning gave Bane and Talia a chance to kill him themselves.
In the mind of LoS, the western civilization had already reached its pinnacle. That it was time to start anew. Ra's first plan was economics, to help show the greed of people and lack of sympathy towards one another. His second plan, was to release the weaponized fear toxin to show how cruel people are to one another. Finally, Talia and Banes plan was to hold siege of Gotham with the hint that they could live if the follow the rules that they had set. There plan is like the pit that Bane lived in and Talia had been born into. Brought on by the injustice of the world where money was power and power was everything. We see this through Dagget, his hunger for more money, teaming up with Bane to take control of Wayne enterprise. The defunding of the At Risk and Orphan Children foundation that directly lend teenagers and young adults into the grip of Bane.
The mob bosses and mofia crime may have been eliminated, not white collar, not random domestics violence. There was still a job shortage and people needing to find work. The part of crime that is harder to predict are crimes of passion. Things that are dictated by the environment that society creates. The need for food, financial troubles that divide families, or even the need to keep ones public reputation clean to stay in power.
The city of Gotham is symbol of the entire western culture. The solution to that is to destroy it. In BB and TDKR, they do this, with different plans, in a way to publicly show the world the need to change, to be anew. In the TDKR, Bane is the byproduct of the corrupt and injustice of the world.