Well there had been clues sprinkled throughout the show that it was possible for Daenerys to become evil. Just you had to have been looking for those clues as the show preferred to completely hide the twist instead of rewarding observant viewers.
Animorphs did the opposite with Rachel. The book series fully acknowledged that she was wavering between succumbing to her bloodlust and staying true to her humanity. So when she made her choice it made absolute sense.
