So the people saying Garrosh is simply a bad apple (even some at Blizzard says this) just can't be correct, why else does he have these visions? If an Old God is granting you visions they are manipulating you, it's as simple as that. They did it to Deathwing and Azshara as well.
By letting them believe in superiority and by showing them a possible future they manipulate these characters to do their bidding. They are the definition of corrupted.