1)
What did the Nazir do wrong that requires him to bring a Chatas on the day that he terminates his Nezirus?
1.
Ramban: Having attained such a high level of Kedushah and Avodas Hashem, he should have remained a Nazir 1 and not returned to his original status. 2
1
Ramban, seeing as the Pasuk in Amos, 2:11 praises and places on a par with a Navi, and seeing that the Torah, earlier, in Pasuk 8, describes him as "Kadosh l'Elokav".
2
Presumably, the Ramban means that, initially, he should have declared himself a permanent Nazir, bearing in mind that, once his term ends, he no longer has the option of remaining a Nazir
