As Parshat Naso unfolds, one sees many different types of separation from the Jewish people; namely סוטה, נזיר צרוע, זב, טמא לנפש. It is interesting to observe that until the Nazir, all of the separations were made because the people had sinned and were being punished. The Nazir, however, separates himself because he wants to come closer to Hashem, and not because he had sinned. However, if the purpose of the Nazir separating himself was to come closer to Hashem, why does the Torah state והקריב את קרבנו, "And he shall offer his offering..." for a sin offering?
The Rambam therefore states that the Nazir did sin. The Nazir has added prohibitions to his life. The Rambam says that Hashem has already given the Jewish people enough prohibitions, and it is a sin to add more. This is why the Nazir brings a Chatat. This view is hard to understand because if the Nazir had sinned, why does the Torah say about him:קדוש הוא לה' , "He is holy unto the Lord?"
To understand a reason for the Chatat being brought, one can combine the answers of the Ramban and Rav Hirsch.
The Ramban states that when the Nazir took an oath of Nizirut, he promised Hashem that he would not do certain things. When he finishes his Nizirut, he will once again do the actions that he promised he would not do, and these actions lead him to sin. It is for this reason that the Nazir brings the Chatat at the end of his Nizirut; because he will most likely commit transgressions against Hashem in the future.
Rav Hirsch states that usually the Chatat is brought before the Olah. Whenever the Chatat comes before the Olah, it is because a sin was committed. Whenever the Chatat comes after the Olah, it is to warn someone not to commit any future sins.
Possibly, now one can explain the reason for the Chatat being brought by the Nazir. As one becomes part of a community, he cares more about what the community thinks and he cares less about his actions towards Hashem. The Nazir brings the Olah first because he wanted to become closer to Hashem on a personal level, and separate himself from the community. He then brings the Chatat because after he finishes his Nizirut, he will once again enter the community and care about vain matters such as how his hair looks, and he will do things because of the community, such as attending parties and drinking wine. He may thus commit sins because of these actions. As Rav Hirsch and the Ramban say, he brings the Chatat so he will not sin in the future. He finally brings the Shlamim, a Korban Tzibur, to be shared, because he will now once again join the community. May Klal Yisrael have the ability to follow Hashem's ways rather than follow the ways of other people, and then possibly we will merit Mashiach coming Bimheirah Biyameinu, amen.