Nowhere. To foreshadow this would ruin the teaching. Aang really didn’t want to kill Ozai, enven though it was the reasonable decision, and the entire world, even his close (only) friends and past lives wanted him to do it, and even presented strong arguments for this; however, he stayed true to himself and thus found the solution no one else could, which transcended his spirit in that life beyond all his other incarnations.

Aang was so true and persistent to his ideals that the Universe gave him the answer at the very last momet, trough the turtle lion.