Law Case: If X administers poison to Y who becomes ill and would have died in two hours' time but 2 minutes prior to Y's death, Z cuts off Y's head..... Who is responsible for Y's death?

If X administers poison to Y who becomes ill and would have died in two hours' time but 2 minutes prior to Y's death, Z cuts off Y's head.....
Who is responsible for Y's death?

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