I think that the main idea of "Dorothy Must Die" is to always do something that you think is right and not get pressured into it and don't do it to please other people do it to please yourself. I think that this is the main idea because...
In the story Amy Gum (the main character) when she arrives in Oz she doesn't know what to do but she just continues on the path. On her journey she meets a wingless monkey named Ollie. Ollie is being punished for something that Dorothy didn't like. Amy wanted to free him because she saw how helpless he looked and she saw him in his worst condition, him nailed to a wooden post. She freed him because it's what she thought was the right thing to do in her position although her company who was used to this sort of thing said not to because she knew the trouble that would come out of it.