Having said "Al Nefesh Mes Lo Yavo", why does the Torah see fit to add Aviv, Imo, Achiv and Achoso?
Rashi (Berachos, 19b): The Torah adds 'Aviv', to preclude a Meis Mitzvah from the prohibition; 'Imo', [to permit a Meis Mitzvah] even when the Nazir is a Kohen; 'Achiv', even if he is a Kohen Gadol, whereas 'Achoso" comes to obligate someone who is going to slaughter his Korban Pesach or to perform B'ris Milah, to bury a Meis Mitzvah and to forego the Mitzvah he is on his way to perform.
The Sifri and Gemara (Nazir 48b) learn from our verse that he may be Metamei for a Mes Mitzvah. We should know this from a Kal v'Chomer from a Kohen Gadol, whose Kedushah is permanent!
Moshav Zekeinim: Shimshon refutes the Kal v'Chomer. His Kedushah was permanent [yet he was allowed to become Tamei Mes]. 1
This is like R. Yehudah (Nazir 4b). Seemingly, R. Eliezer (Nazir 47a) refutes the Kal v'Chomer. A Kohen Gadol does not bring a Korban for Tum'ah! Therefore if a Kohen Gadol and a Nazir found a Mes Mitzvah, the Kohen Gadol should be Mitamei, and not the Nazir! Chachamim disagree, but they should agree that the Kal v'Chomer is refuted! (PF)