Originally Posted by The Infernal
And? It's the part that's important not top billing.
Actually it's the part that people remember that's important. When there comes Avengers 2 people are most likely looking forward to Thor, not Loki. In most likelihood it's the protagonist role that's more popular than the antagonist role.
Heath Ledger's Joker, Darth Vader and Agent Smith of the Matrix are the rarity. IM Obadiah, IM2 Whiplash, Red Skull, Lex Luthor, etc are the norm.