gentiles - Did Kozbi deserve to die?

In the end of Parshas Balak (Bamidbar 25:8), Pinchas kills an Israelite who was cohabiting with a Midianite woman in front of the Jewish people.

I understand that the Israelite deserved to be killed as per the halacha of בועל ארמית קנאין פוגעין בו, one who cohabits with a non-Jewish woman is open to be struck by a zealous person. However, did the Midianite woman do anything punishable by death? Cohabiting with a Jew is not on the list of Noahide prohibitions.

One possible approach would be to assume that gentiles have a prohibition of "לפני עיוור", causing others to sin. This would require assuming that violation of said prohibition would be punishable by death. However, some (most?) opinions hold that there is no such prohibition for gentiles (see S'dei Chemed Aleph 26:23).

That being the case, why was killing Kozbi (the Midianite woman) justified?

Answer

Good question.

Or HaChaim (Bamidbar 25:8) asks the same question, and answers that she had the דין of the animal involved in bestiality -- "ואת הבהמה תהרוגו," "and you shall kill the animal" (Vayikra 20:15).

וידקר את שניהם וגו'. קשה בשלמא דקירת איש ישראל כמשפט ההלכה, שקנאים פוגעים בו, אבל האשה אינה חייבת מיתה ואינה מצווה, ואם על חששת היותה אשת איש לא היתה נהרגת על הספק. ואפשר שדן בה משפט הבהמה דכתיב (ויקרא כ:טו) ואת הבהמה תהרוגו, וכתיב (יחזקאל כג:כ) אשר בשר חמורים בשרם

"And he pierced both of them" -- one might ask, that the piercing of the man makes sense because he sinned and was killed according to the rule of law ("קנאים פוגעים בו"); but why was the woman killed? Her action did not require capital punishment; and if there was a concern that she may have been married to another man, she should not have been killed just due to this concern, because it was in doubt ("ספק").
Maybe she was given the same ruling as the animal involved in bestiality, as the verse says "ואת הבהמה תהרוגו" "and you shall kill the animal."

The footnote in my Mikraos Gedolos Chumash (quoting נר למאור) led me to the Rambam (איסורי ביאה 12:10) who says the same thing [though he leaves כזבי out of the picture, he quotes the later verse that commands the death of all the Midianite women who were involved in this scandal]:

אבל ישראל הבא על הכותית בין קטנה בת שלש שנים ויום אחד בין גדולה בין פנויה בין אשת איש ואפילו היה קטן בן ט' שנים ויום אחד כיון שבא על הכותית בזדון ה"ז נהרגת מפני שבא לישראל תקלה על ידיה כבהמה ודבר זה מפורש בתורה שנאמר הן הנה היו לבני ישראל בדבר בלעם וכל אשה יודעת איש למשכב זכר הרוגו

...she (the non-Jew involved with a Jewish man) is killed because a תקלה (disaster) came about because of her, and she shares the ruling of the animal. This rule is explicit in the Torah [in the verse that commands the deaths of the other Midianite women]

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It should be noted that this is only a technical, exegetical comparison, and not a philosophical one -- we don't think that non-Jews are animals. (Kudos to Fred for reminding me to clarify this point.)

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^{All translations my own.}