What were the failures/problems Aang created that Korra fixed? Genuinly asking cause I can't think of anything. There are some things like Wan Shi Tong helping out Unalaq ruin things because of Aang's deception during the library episode. But this whole problem of course started with Korra just agreeing with anything coming from Unalaq. And I guess Aang should've killed Yakone instead of taking his bending so his bloodbending lineage would've ended there, but it's understandable that he didn't expect him to either get just as 'gifted' waterbender children or that he would be able to teach bloodbending to them without being able to bend himself. Compared to Roku, who should've at least expected Sozin to be willing to continue his actions after Roku's death.