For hadn't they been following a plan? Laid out by Dumbledore, wrapped in allegory and misty deceit. Horcruxes, Hallows, Prophecies ... and an end to the darkness of Voldemort. Harry was the Chosen One, the Boy Who Lived, the one destined to vanquish the evil of Tom Riddle. Hermione simply couldn't wrap her head around him just walking blindly to his death as she'd been told. He wouldn't do that. There must have been a reason. He was her best and closest friend, and she refused to believe that he would have just left her all alone.