Re: Why did the League of Shadows want to destroy a peaceful city?
I know I'm a little late on this topic, and someone may have already said this, but the League of Shadows only attacked Gotham because of Talia. Talia took control of the LoS (being the heir to the throne and all) and used Bane as her muscle to take down the city her father dedicated his life, and died, trying to take down. Talia didn't care if Gotham was clean or not, she just wanted her father's life goal to be complete.
"A Man Remembers His Debts"