What made Daryl kill her Sister?

This movie made him go crazy.

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Might want to spoiler tag this

The Hat lol. They all lost it I think, he was just the first one to "snap"

The woods and the music made him do it. He was the first person to snap. It eventually affected the rest of the team one by one throughout the course of the movie. One example was when Cy told Liz that he is starting to experience what Daryl was experiencing and that eventually he will be killing Liv unless he ties him up.