I always interpreted her death in the car accident as a way to help Daryl progress. To him, she was basically another "addiction" he was slipping into to avoid facing his own problems. The only real way to give him "closure" was to do something with her character that he could not undo. Then, he would have no choice than to either give up and go back into drugs or finally make the commitment to being clean and sober. I think that was the entire point of the film!

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