Well, if you want My take on it... I say Courtney IS an antagonist
Because while she's actually one of my favorites (Sheids herself from fruit being flung at her), the word "Antagonist" really means a person who gets in the way of the protagonist. Much of the time the term is used along with the word "villian" which is why the definitions are sometimes mixed up. Anyway, seeing as how Courtney pretty much has a bone to pick with almost everyone (Except maybe Duncan), then technically I can't argue that she's not the season's antagonist, and that Justin was a red herring until her return.
That said, I'm still not convinced yet that she can actually be described as evil. Selfish? Yes, very much so. But ask yourselves this; if Chris had the contestents do an apocolyptic movie challenge, and there was an earthquake obstacle, and Duncan was about to fall into a crack, along with an invincibility pass falling right next to him, which would Courtney grab first?