Why do Penny and Primrose decide to leave Alys behind?

Penny and Primrose decide to leave Alys behind because they realize that her presence is hindering their progress and safety. Alys's behavior has become increasingly erratic, creating tension and uncertainty within the group. Ultimately, Penny and Primrose prioritize their own survival and the need to move forward, believing that they can no longer rely on Alys. Their decision reflects a harsh but necessary choice in a difficult situation.