http://revach.net/article.php?id=2467

Section: Parshas HaShavua   Category: Quick Vort
Parshas Pinchas: Rav Moshe Feinstein - What It Takes To Be a Kana'i

Pinchos got up and decided on his own to defend the honor of Hashem and kill Zimri.  He acted in accordance with the halacha of "Kana'im Pog'im Bo".  Had he asked Bais Din for permission, he would not have been told to kill him.  A Kana'i cannot get permission from Bais Din.  He must act on his own.  This halacha is quite unique and contrary to the general Torah Hashkafa. Why is this so?

Rav Moshe Feinstein says in Darash Moshe that while it is important to do mitzvos with pure intentions, nevertheless, a mitzva done with ulterior motives and for the sake of personal benefits, is still worth doing.   However, when it comes to punishing others, this can only be done if carried out with 100% pure intentions and totally L'Shem Shamayim.   Judging a person's intentions are not within the abilities of another human.  Only Hashem can know a person's thoughts.  Therefore, in the case of Kana'im Pogim Bo, Bais Din can never render an opinion if a particular person can carry out "Nikmas Hashem".  This is between the person and Hashem alone.

This was the gripe of the Shevatim against Pinchos.  They doubted his sincerity until Hashem testified on Pinchos's behalf, and awarded him the Kehuna and a Bris of Sholom.