1)
What did the Nazir do wrong that requires him to bring a Chatas on the day that he terminates his Nezirus?
1.
Ramban: Initially, he should have declared himself a permanent Nazir, to remain at such a high level of Kedushah and Avodas Hashem 1 , and not returned to worldly lusts.
1
Ramban, seeing as the Pasuk in Amos, 2:11 praises and places himon a par with a Navi, and seeing that the Torah, earlier, in Pasuk 8, describes him as "Kadosh Hu la'Shem".