Nazir 43: The Beheaded Kohen

What verse teaches the limitations on the kohanim about coming into contact with a dead body? The kohanim not only are not supposed to come in contact with impurity, and then they do, they also engage in a measure of profaning their extra holy status (it's not a bad thing, just a thing). Plus, a dispute based on the nuance between two verses, as per the differences between a kohen and nazir as well. Also, if a man's head was cut off, a kohen cannot become impure for that person, even when related, because the body needs to be intact for a kohen to be allowed to be impure for him. With exceptions, especially a "met mitzvah."