The sin offering is brought by the Nazir because he chooses to step down from such a high level of holiness, therefore he is a sinner, we always go up in Kodesh not come down. as the RAMBAM states that a Nazir's level of holiness is higher than a High Priest in Yisrael, where is should have taken the vow FOREVER Le-Olam and stay Kadosh all his life without having to bring any of the sacrifices

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