Leviticus 21:1

AND the Lord spake to Mosheh, saying: Speak unto the priests, the men of the children of Aharon, that they keep themselves apart from defilement and thus shalt thou say to them: For a man who is dead, (the priest) shall not defile himself among his people;

Speak to the kohanim After commanding the Children of Israel (including the children of Aaron) to be holy [19:2], Scripture now commands the children of Aaron to observe additional restrictions, because they are dedicated to God’s service.

It is also possible that Speak to the kohanim means: Teach the kohanim the previous parasha — because the propagation of the Torah has been entrusted to them — whereas and say to them refers to the upcoming commandments, which they alone must follow.

with a soul this means, with a dead person.

defile himself This verb is in the hitpa‘el form. Its ṭet acquires a dagesh because it absorbs the preceding tav .

among his people among all Israel, which is his nation.

SAY [UNTO THE PRIESTS] … AND THOU SHALT SAY UNTO THEM. “[The repetition of the verb say … and thou shalt say is] to warn the adults about the children” [that they observe this law]. This is Rashi’s language on the basis of the words of our Rabbis. Now the meaning of this “warning” is to state that we are not to assist with our hands in the defilement of children [but it does not intend to warn us that we are obliged to prevent them from becoming defiled]. There are many admonitions in the Torah of this nature according to the interpretation of our Rabbis, such being the prohibitions against eating blood and swarming things, and against the defilement [of priests], and from them we learn that [it is applicable to] all prohibitions of the Torah, that we must not assist children in any case to transgress the law. But if they are doing it of their own accord, we, and such is the final decision of the law, Shulchan Aruch, Hilchoth Shabbath, 343). are not commanded to prevent them [from so doing since they have not yet reached the training period of minors for religious practices, but if they have reached that stage, everyone is commanded to prevent them from doing a prohibited act]., Rama, and Mishnah Brurah, 7. The meaning of the verse according to its interpretation is then: “say unto the priests … and thou shalt say it over again to them, that they are not to defile themselves,” the purpose of the many warnings being [to teach] that all sons of Aaron are to be guarded from defilement, even the young ones. And Rabbi Abraham ibn Ezra said by way of the plain meaning of Scripture, that say unto the priests refers to the section mentioned above [i.e., Seder Kedoshim], since they [the priests] are the teachers of the Torah and it is they who admonish the people; and thou shalt say unto them, meaning those specific commandments which devolve upon them alone to observe, [as mentioned in this section]. But it is not correct [to interpret two such adjacent phrases as referring to two different sections].
In my opinion the plain sense of the verse is as follows: The meaning of emor (say) is like that of dabeir (speak): Give ear to ‘amarai,’ O Eternal is like d’varai (my words). A similar case is ‘imrei’(words of) truth. (words of truth) — Ecclesiastes 12:10. So also: for it hath heard all ‘imrei’ (the words of) the Eternal. (the words of G-d) …

לנפש לא יטמא בעמיו, a man belonging to the “nation” of priests must not defile himself by contact with a dead human being. The word בעמיו “of his people,” is used here in the same sense as in verse 14 later on where the Torah decrees that the only suitable marriage partner for a High Priest [if he was not married at the time of his appointment. Ed.] is בתולה מעמיו, “a virgin from his people.” If the Torah had written here לעמיו instead of בעמיו, the meaning would have been more restrictive, i.e. that the priest must only not defile himself by contact with a Jewish corpse. Similar constructions involving the expression לעמיו are found in Numbers 6,7 The expression לנפש allows leeway, as we know from the subsequent verse where it is permitted for the priest or even mandated that he defile himself on the corpses of his closest relatives.

ויאמר ה' אל משה אמור אל הכהנים, what has been written earlier so that they will understand and instruct each other in the various categories of ritual impurities and contaminations, including the difference between ritually pure domestic beasts and birds, for these are the aspects of foremost concern to the priests on a regular basis as we read in 10,10-11 ולהבדיל בין הקדש ובין החול, ובין הטמא ובין הטהור ולהורות, “and to separate the sacred from the profane, and the ritually impure, and between the ritually contaminated, and to teach these values.”

ואמרת אליהם, that the need to exercise caution in addition to these categories of the ritually pure and that which is not, also in regard to impurity conferred through contact with the dead as well as through desecrating their seed.

לנפש לא יטמא בעמיו, no one of the priestly caste must ritually defile himself through dead bodies belonging to his nation., i. e. any dead person who is not one of the seven family members for whose sake he is to ritually contaminate himself to arrange for their burial, etc.

[לנפש לא יטמא בעמיו], “he must not ritually defile himself on account of the dead [members] of his people.” If he cannot defile himself in order to bury his relatives, how much less may he do so for people not related to him!