Re: Why did the League of Shadows want to destroy a peaceful city?
Talia was an ex-member? When did she leave?
Talia was the heir and after Ra's died we can assume that the League fell into her hands.
The mercenaries in TDKR wore the same clothes as the members in the temple in Batman Begins (who yes, had guns). Not all the League are ninjas. They seemed to be not just 'hired guns'. ("a lot of loyalty", remember? "have we started the fire", etc)
Since Talia is the leader, she can reinstate Bane into a position of power and respect.