The גמרא says that the words “I will be נאה” can be a קבלת נזירות because even רבי אלעזר הקפּר who said that a נזיר is considered a חוטא only said that by a נזיר טמא. If you look back at נדרים דף י you will see that while it is true that רבי אלעזר הקפּר learned that a נזיר is a חוטא from the פּסוק by a נזיר טמא, his reasoning is that the person is a חוטא is because he was מצער himself from wine which לכאורה would apply to all נזירים. The תורה תמימהon that פּסוק suggests that perhaps this דרשה was said by a נזיר טמא since that is an extreme example of צערsince the person never wanted that length of a נזירות to start with so now he is really מצער עצמו. Regardless, it seems that would say that a נזיר is considered a חוטא even by a נזיר טהור. תוספות in ד"ה ואמאי answers that both are really true—a נזיר טהור is both a good and bad. פּרישות is good but being מצער yourself is not good. On balance, its more positive than negative so in that sense a נזיר טהור is not a חוטא. Nonetheless, there is some element of חטא. תוספות compares this to someone who fasts on Shabbos for a תענית חלום where the תענית is a כפּרה yet he needs to accept upon himself another fast because of the sin of fasting on Shabbos.

The מרומי שדה here finds this פּשט very difficult. How can something be a מצוה and עברה all at once? As to the proof from תענית חלום, he says that it’s פּקוח נפש and of course its a מצוה to fast on Shabbos in that case. The reason he has to do the second fast is that if Hashem made him come to חילול שבת because of this dream it means he had sinned before Shabbos and he needs כפּרה for that sin! To answer the question about רבי אלעזר הקפּר, he explains that a נזיר that became טמא is a sign from Hashem that he wasn’t ראוי to be a נזיר. So he is a חוטא for being מצער himself. However, a נזיר טהור did nothing wrong by being מצער himself. This is similar to בעל תשחית, where we would say someone who throws out a very fattening piece of cake because it’s not healthy is not violating בעל תשחית since, as the גמרא says in שבת, בעל תשחית דגופא עדיף (destroying your body is a worse type of בעל תשחית). Similarly, here, its better for his soul to be מצער himself if he is ראוי so he is not a חוטא at all.

The גמרא quotes רבי שמעון who says you aren’t a נזיר until you are מקבל on yourself all aspects of נזירות and the חכמים disagree and say that if you are מקבל to be a נזיר to not drink wine, you still can’t cut your hair or go to a cemetery. There is a widely quoted מנחת חינוך in מצוה שס"ח who questions whether according to the חכמים if someone said they are only a נזיר for not drinking wine and then cut their hair, would they violate בל יחל דברו. After all, their words never said they wouldn’t cut their hair (a regular נזיר does violateבל יחל since by saying he is a נזיר all איסורים are included). The קהילת יעקב in סימן ב proves from the רמב"ם that he would get מלקות. He brings the רמב"ם in הלכות נדרים פּרק ה that holds that if you make a נדר that your friend can’t get הנאה from you and then you feed him food, you violate בל יחל. In that case you didn’t say I won’t give him benefit; you said he cant get benefit. Yet you violate בל יחל. It must be that its not your words themselves but the איסור that comes from your words that causes the בל יחל.