The other answers are good, but I'll add a perspective derived from a close reading of the Hebrew.
First, a note on the syntax. To my mind, any translation that runs "he shall not make himself unclean as a [noun] among his people" is hard to defend based on the Hebrew we have, even if the older commentators' Vulgate or Septuagint translations might support it. Here's an interlinear:
לֹ֥א יִטַּמָּ֖א בַּ֣עַל בְּעַמָּ֑יו לְהֵ֖חַלּֽוֹ׃
not / shall-make-self-unclean / a-ba'al / among-his-people / so-as-to-pollute/profane-himself
The only way to read this Hebrew sentence is with ba'al as the subject:
A ba'al among his people shall not make himself unclean, thereby polluting/profaning himself.
Here are the possible interpretations, still leaving ba'al untranslated:
(1) A ba'al (the priest) shall not make himself unclean through his people.
(2) A ba'al (the priest) shall not make himself unclean among his people.
(3) A ba'al among his people (the priest) shall not make himself unclean.
(4) A ba'al among his (the priest's) people shall not make himself unclean.
I'll translate Lev. 21:1-3 directly from the Hebrew to get some context:
וַיֹּ֤אמֶר יְהוָה֙ אֶל־מֹשֶׁ֔ה אֱמֹ֥ר אֶל־הַכֹּהֲנִ֖ים בְּנֵ֣י אַהֲרֹ֑ן וְאָמַרְתָּ֣ אֲלֵהֶ֔ם לְנֶ֥פֶשׁ לֹֽא־יִטַּמָּ֖א בְּעַמָּֽיו׃ כִּ֚י אִם־לִשְׁאֵר֔וֹ הַקָּרֹ֖ב אֵלָ֑יו לְאִמּ֣וֹ וּלְאָבִ֔יו וְלִבְנ֥וֹ וּלְבִתּ֖וֹ וּלְאָחִֽיו׃ וְלַאֲחֹת֤וֹ הַבְּתוּלָה֙ הַקְּרוֹבָ֣ה אֵלָ֔יו אֲשֶׁ֥ר לֹֽא־הָיְתָ֖ה לְאִ֑ישׁ לָ֖הּ יִטַּמָּֽא׃
YHWH said to Moses, "Speak to the priests, the sons of Aaron, and say to them: 'By a corpse he shall not make himself unclean through/among his people, except by his kin, his close relative — by his mother or his father or his son or his daughter or his brother or his virgin sister, his close relative, being that she has no man; by her he shall make himself unclean.
Some observations. One, the referent of "he" is only implicitly one of "the priests"; no clear subject is given. This is the case throughout the chapter. However, a strong sign that it means "priest" is in verse 8: "he offers the food of your God". Because of that, I'm comfortable ruling out (4). That is, ba'al refers to the priest. The only alternative is that the subject switches from "he (the priest)" to "some other ba'al" for verse 4 and then back.
Two, in verse 1 we have "he shall not make himself unclean among his people". This use of the phrase "among his people" shows that it's attached to the verb, not to ba'al. That rules out (3). This is about a ba'al pure and simple, not a relative epithet like "a chief man among the people".
Our next decision is whether he makes himself unclean among his people or through them. I'm inclined to say through because it's one of their corpses by which he would make himself unclean. We might also suppose that the extensive list of family members is meant to be those who are exempted from "his people"; they're his kin, not his congregation, making verse 4 a reaffirmation.
Besides, what would it mean to be "unclean among one's people" that differentiates it from simply being "unclean"? Note that we can't stretch "among" to "in their eyes", for which there's a different expression. We're already stretching it from a literal "in/by" to "among".
That leaves us with syntax (1): "A ba'al shall not make himself unclean through his people."
The final decision is, of course, how do we translate בַּ֫עַל ba'al?
The three main branches would be:
chief, lord, master: This priest is the (spiritual) leader of the people. (Incidentally, ba'al is used in the names of many local deities, which might add that spiritual nuance - "regard him as holy" - but that isn't key to this reading.) This seems like the most natural reading to me.
steward: That is, the priest is charged with taking care of the people. Compare 1 Kings 18:3 where Obadiah is עַל־הַבָּיִת "in charge of the palace". This may require a stronger connection to ba'al's etymological roots than is warranted, but it does mean "owner" in other places, with the associated responsibilities.
husband: I see two possibilities here:
- A) The priest is figuratively a husband of the people (cp. the Church as the bride of Christ, who is, after all, "a priest" according to Hebrews). Although this seems interesting and also calls up analogies of God as husband in numerous prophetic passages, I think it would be anachronistic to call that the primary meaning — what the author had in mind.
- B) This injunction applies to the priest insofar as he is also a husband.
Clearly, the NIV chooses this final reading. The logic apparently runs that the author refers to the priest as a husband for one verse, and by implication refers to those of his kin who are related to him through marriage. I'm far from convinced. Or else they have misread the syntax as: "He (the priest) shall not make himself unclean, [as a] a husband among his people." As I said at the beginning, this is unidiomatic in Biblical Hebrew, perhaps even ungrammatical.
Therefore, whichever nuance of ba'al between leader, steward, or spiritual spouse we choose to focus on, I propose reading the verse as a summary or reaffirmation of the end of verse 1:
A leader shall not make himself unclean through his people.
Now why talk about leaders here instead of just "he" as with the other verbs in this chapter? I'm not sure we can know, but I'd guess that this particular statement is meant to be true of priests because they're a subset of leaders: "The priest won't make himself unclean through his people — leaders shouldn't do that."
P.S. If I were a commentator I'd wonder if the whole paragraph didn't basically mean: "Your priest has to stay ritually pure too. He's not going to be responsible for everyone's dead, just his own family!"