kana'im pogim bo and the need for hasra'ah

Why does the Torah mention the yichus of Pinchas at the opening of our parsha?  Rashi explains:

לפי שהיו שבטים מבזין אותו: הראיתם בן פוטי זה, בן שפיטם אבי אמו עגלים לעבודה זרה יהרוג נשיא שבט מישראל, לפיכך בא הכתוב ויחסו אחר אהרן.

What does the fact that his grandfather Yisro fattened calves for avodah zarah have to do with the price of tea in China?  

Some explain that the shevatim could not believe that Pinchas' zealotry was completely l'shem shamayim.  They thought maybe he was just a hothead.  There are people who are makpid on details and if even one little thing is slightly off, they blow up.  They can't delegate, because they have tolerance for anyone making the smallest error.  This is what the shevatim meant פיטם אבי אמו עגלים לעבודה זרה.  Why was Yisro, the highest priest, fattening the calves himself?  It must be that he was one of these people who needed to personally handle everything, one of these people who blew up at the drop of a hat if the slightest thing is off.  Is it any wonder that his grandson would be a zealot?  It's in his genes, not l'shem shamayim!  Therefore, the Torah stresses that Pinchas was the grandson of Aharon, the ohev shalom and rodef shalom.

I often daven in a Moroccan yeshiva and have the opportunity to see seforim and sources there that I never heard of before, which is how I chanced upon the Maggid of Marrakesh.  The Maggid explains Rashi by quoting the din in Yevamos 102:

 גר דן את חבירו דבר תורה שנאמר שום תשים עליך מלך אשר יבחר ה' אלהיך בו מקרב אחיך תשים עליך מלך עליך הוא דבעינן מקרב אחיך אבל גר דן את חבירו גר ואם היתה אמו מישראל דן אפי' ישראל 

This is what bothered the shevatim -- how could Pinchas, who was descended from a ger, judge a member of Klal Yisrael?  Therefore the Torah responds by telling us the yichus of Pinchas, tracing his lineage to a Jewish father, which solves the problem.  

Granting that this approach is derush, I still think it's noteworthy that the Maggid treats קנאים פּוגעים בּו as requiring the same criteria as a din torah.  It's not an extra-judicial concept, but rather is a form of misas beis din carried out by the individual, who serves as judge and jury wrapped in one.

Perhaps that assumption is the nekudas ha'machlokes between Rambam and Raavad.  Rambam writes in Issurei Biah 1:4

כל הבועל כותית בין דרך חתנות בין דרך זנות אם בעלה בפרהסיא והוא שיבעול לעיני עשרה מישראל או יתר אם פגעו בו קנאין והרגוהו הרי אלו משובחין וזריזין . ודבר זה הל"מ הוא ראיה לדבר זה מעשה פנחס בזמרי:

Raavad adds:

תב הראב"ד ז"ל בד"א שהתרו בו ולא פירש אבל לא התרו בו לא אמרינן הרי אלו משובחין עכ"ל:

There is a very difficult Migdal Oz here:

ואני אומר התראה זו לא מצאתיה לא פרק אין מעמידין ולא פרק הנשרפין בבבלי אך בירושלמי יש לו מקצת טעם דאמרינן במעשה זמרי פנחס שלא ברצון חכמים עשה אלא שאין לנו לסמוך עליו הואיל ולא הוזכר בבבלי ושכרו שנתן לו השם מוכיח וגם ר"י אלפס ז"ל לא הביאו בהלכות לומר לך שאינה הלכה ועוד שהרי זמרי בפרהסיא עשה והביאה לפני משה והתריס ומה התראה גדולה מזו והלא אין התראה באה אלא להבחין בין שוגג למזיד וודאי דברי ר"מ ז"ל נכונים:

He writes that the best proof against the Raavad is that we don't see Pinchas giving any hasra'ah to Zimri, but then backtracks and says perhaps that gufa is the reason that the Yerushalmi says שלא ברצון חכמים עשה.  It sounds like he is suggesting that Pinchas may, according to the Yerushalmi, have been in the wrong!  I don't know how you an say that when the whole parsha is about the tremendous reward Hashem granted to Pinchas for his actions.

Be that as it may, perhaps the machlokes Rambam and Raavad here revolves around the Maggid's assumption.  If קנאים פּוגעים בּו is some kind of extra-judicial penalty, then all rules of judicial process go out the window.  However, if it is some quasi maaseh beis din, then maybe hasra'ah is required similar to other cases of dinei nefashos carried out by B"D.

Rav Yaakov David Ilan, in his sefer Masa Yad vol 1, learns the machlokes differently.  Yesh lachkor, he writes, whether hasraah is a din in B"D or a din in the chomer ha'aveira, i.e. is it is the same aveira with or without hasra'ah, but the court needs hasra'ah as a license for them to act, or whether an aveira without hasra'ah is a lesser aveira, and therefore exempt from punishment.

If hasra'ah is a din in B"D, then קנאים פּוגעים בּו, which is not a maaseh B"D, should need no hasraah.  But if hasraah establishes the chomer ha'aveira, then it is necessary even for the punishment of קנאים פּוגעים בּו. 

See also R' Shaul Yisraeli's analysis of the sugya in his Amud ha'Yemini.