In Red and Blue, it learns Confusion at Level 12, two Levels after it evolves from Metapod. In every other game, it learns Confusion at Level 10, as soon as it evolves from Metapod.
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Butterfree learns confusion at level 12.
If your Pokemon evolves later it could learn more attacks in its pre-evolved form that maybe could not learn if evolved (particularly true with Pokemon who evolve from stones) or it could skip some attacks that should learn in its evolved form. In this case, it is not important, because butterfree will learn Confusion at level 12.
14
Butterfree can't learn Aerial Ace by level up. It can, however, learn it by TM, which you can find on Route 9.
You have to use the TM to teach it Psychic.