sin - Nazir and Korban Chatas

I understand that the Torah orders that at the time that a Nazir finishes his year he is required to bring a Korban Chatas. I'm finding it difficult to understand this. First of all what issur did the Nazir do? The Nazir did a mitzvah by taking upon himself the nazirite oath. And secondly even if you want to find a way to say that the Nazir needs atonement, a Korban Chatas is only for someone that sins by accident, an example would be if a person forgot it was the sabbath and did one of the 39 biblical sabbath prohibitions by accident. However in this case, the Nazir knowingly and willfully takes upon himself the nazirite oath. So why does he bring a Chatas?