I believe that Snape is the good one who seems to be traitorous. He was only fulfilling the orders of Albus Dumbledore as he was ordered to do near the edge of the forest (which was overheard by Hagrid).

The thing that will totally blindside everyone is that Minerva McGonagall will be the bad one who seems to be the most loyal to Dumbledore. She acts a bit odd after each major event and her emotions either seem out of place or there is a total lack of emotion. McGonagall was very close to DD and could be the "most faithful servant" to Voldemort at Hogwarts as the Dark Lord mentioned during the graveyard scene in GoF while taking attendance with the Death Eaters.