To save the life of a child, Elizabeth had to take a life. She killed Daisy Fitzroy, leader of the Vox Populi. The Elizabeth we refused to dance with at the docks can't stomach what she has done, no matter how noble the act. It's a fairly simple visual metaphor. The childish, innocent Elizabeth can no longer be, so she sheds her long ponytail and changes her clothes. It's a shocking, daring change in character, and it's where we start to see the tragic nature of Elizabeth's lot in life.