Thoughts on Liberty made a great post explaining why Disney completely changed the story arc in FROZEN to create one of the more versatile characters ever seen in Disney films: Elsa, who was originally a strict villain archetype.
What inspired the change? A song, of course!
[W]hen the character’s major song, “Let it Go,” was played for the producers, they concluded that the song was not only very appealing, but its themes of personal empowerment and self-acceptance were too positive for a villain to express. Thus, the story was rewritten….
If you re-watch early advertising for the film, it definitely seemed to suggest Elsa was much more evil than what actually occurred in the film. That’s why!