Aang didn't leave a peaceful world for Korra. He favored benders, created Republic City out of Earth Kingdom territory, and ordered the White Lotus to guard the next Avatar which splintered into the Red Lotus.
This created Amon, Zaheer, and Kuvira as villains in Korra's era who were all upset at things Aang had done.
Avatar Szeto focused on the fire nation and left Yangchen to deal with the political imbalance.
Avatar Yangchen focused on the human world and left Kuruk to deal with the dark spirits.
Avatar Kuruk focused on dark spirits and left Kyoshi to deal with the political imbalance.
Avatar Roku couldn't kill his best friend and left Aang to deal with the 100-year war.
Avatar Korra, for whatever reason, left an apocalyptic world for Pavi to deal with.
Avatar Kyoshi and Avatar Aang are the only ones to have left a relatively peaceful era for their predecessor, which is why I hold them in high regard as the best Avatars in history.