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Essential Raid Arc:
They had no way to stop the cube from taking effect, Lucifer being immortal, he didn't pay attention to what was happening to him, and he was worried about Lynx, he was just a simple man with some supernatural powers.
Lynx realized that he could not stay alive while being tied to the basement wall by Jin. once he found out about the sheet, and the card, where them are hiding, it was normal to kill him too, he would not let him hang and wait for help.