Only a pure and unblemished sacrifice could atone for the sins of the world. Hence Christ could not have inherited original sin.
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So why didn't they crucify Mary? According to RCC beliefs, she had no original sin.
Being without original sin is not sufficient to be an atonement for the sins of the whole world. Anselm in Cur Deus Homo show why the redeemer had to be God. Mary was not God; she was human. And therefore, Mary could not be the one whose death takes away the sins of the world. Morover, Mary was sanctified at her conception by the grace of Christ. But the redeemer cannot be in need of redemption.
-A8