And G-d spoke to Moses, saying: “Pinchas, the son of Elazer, the son of Aaron the Kohen, turned away My wrath from the children of Israel, with his zealotry for My sake... Therefore... I shall grant him My covenant of peace..."
-- Numbers 25:11-12
Pinchas' deed evokes many associations - courage, decisiveness and religious passion are several that come to mind - but “peace” hardly seems one of them. Pinchas, after all, killed two people. True, what he did was allowed, and condoned, by Torah law, and his doing so saved many lives; still, one does not usually think of homicide as a peaceful act.
As the Torah tells it, the wicked prophet Bilaam, having failed to undermine Israel's special relationship with G-d by harping on Israel's past sins, had an idea. “Their G-d abhors promiscuity,” he said to Balak, the Moabite king who had hired him to place a curse on Israel. Corrupt them with the daughters of your realm, and you will provoke His wrath upon them.
This time Bilaam succeeded. Many Jews, particularly from the tribe of Shimon, were enticed by the Midianite harlots who descended upon the Israelite encampment in the Shittim valley, and were even induced to serve Baal Pe'or, the pagan god of their consorts. When tribunals were set up by Moses to try and punish the idolaters, Zimri, the leader of Shimon, sought to legitimize his tribe's sins by publicly taking a Midianite woman into his tent, “before the eyes of Moses and the eyes of the entire community of Israel.”
Moses and the nations' elders were at loss at what to do. Torah law does not provide for any conventional, court-induced punishment for such an offender. There is a law that gives license for “zealots to smite him,” but this provision escaped Moses and the entire Jewish leadership. Only Pinchas remembered it, and had the fortitude to carry it through. He killed Zimri and the Midianite, stopping a plague that had begun to rage as the result of G-d's wrath against His people.
The Grandfather Issue
The Talmud, referring to G-d's opening words to Moses quoted above, asks: The Torah has already told us who Pinchas is, back in the sixth chapter of Exodus and again, but a few short verses before, in Numbers 25:7. Why does the Torah again refer to him as “Pinchas, the son of Elazar, the son of Aaron”?
Rashi, quoting the Talmud and Midrash, explains:
“Because the tribes of Israel were mocking him, saying ‘Have you seen this son of the fattener, whose mother's father fattened calves for idolatrous sacrifices, and now he goes and kills a prince in Israel?!’ Therefore, G-d traced his lineage to Aaron.” (Pinchas' maternal grandfather was Jethro, who prior to his conversion to Judaism, was a pagan priest).
This explanation, however, seems to raise more questions than it answers:
(a) What set “the tribes of Israel” against Pinchas? The animosity of one tribe, the tribe of Shimon, would be understandable: he killed their leader and put an end to their pagan orgy. But why was he condemned by the entire community of Israel, most of whom were outraged by Zimri's act and were doubtless grateful for Pinchas' stopping the plague?
(b) Of what possible relevance is Jethro's past? If Pinchas acted wrongly, then he is guilty of much worse than having a grandfather who fattened calves for slaughter. “Murderer” would be more apt than “fattener's grandson.” And if it was acknowledged that killing Zimri was the right thing to do, why was the young hero and saviour of his people being mocked?
(c) If, for whatever reason, Pinchas is to be faulted because of Jethro's idolatrous past, why dwell on the fact that he “fattened calves for slaughter”? What about the fact that he was a pagan priest who had “served every idol in the world”? 
(d) Whatever the complaint against Pinchas was, how is it refuted by the fact that he was Aaron's grandson?
Who Is A Zealot?
The nature of Zimri's crime made his killing an extremely sensitive moral issue. On the one hand, the Torah deems what he did as deserving of death. On the other hand, it does not entrust the carrying out of the sentence to the normal judiciary process, ruling instead that “zealots should smite him.” Who, then qualifies as a zealot? When a sentence is carried out after the “due process” of a trial and conviction, there is less of a need to agonize over the motives of the judges and executioner: they're going by the book, and we can check their behavior against the book.
But the motives of the zealot who takes unilateral action are extremely important, for his very qualifications as a “zealot” hinge upon the question of what, exactly, prompted him to do what he did. Is he truly motivated to still G-d's wrath, or has he found a holy outlet for his individual aggression? Is his act truly an act of peace, driven by the desire to reconcile an errant people with their G-d, or is it an act of violence, made kosher by the assumption of the label “zealot”?
The true zealot is an utterly selfless individual - one who is concerned only about the relationship between G-d and His people, with no thought for his own feelings on the matter. The moment his personal prejudices and inclinations are involved, he ceases to be a zealot.
Thus, the above questions answer each other. The “tribes of Israel” knew that the case of Zimri warranted the law that “Zealots smite him.” But they were skeptical of Pinchas' motivations. Why is it, they asked, that no one--not Moses, not the elders, no one in the entire leadership of Israel - was moved to assume the role of zealot, save for the Pinchas, “youngest of the band”? Was Pinchas the most caring and selfless one of them all? Far more likely, said they, is that what we have here is an angry young man who thinks he found a Torah-sanctioned outlet for his aggression.
A bit of digging around in the skeletons of the Pinchas family closet only re-enforced their initial doubts. Of course! Look at his grandfather! Few professions are as inhumane as the fattening of calves for slaughter. The fact that Jethro was an idolator is not what is relevant here, but his nature and personality - Pinchas must have inherited his grandfather's natural cruelty, and proceeded to clothe it in the holy vestments of zealotry.
So G-d explicitly attached Pinchas' name to Aaron, the gentlest, most peace-loving man that Israel knew. Aaron, the “lover of peace and pursuer of peace, one who loves humanity and brings them close to Torah.” In character and temperament, G-d was attesting, Pinchas takes after his other grandfather, Aaron. Not only is he not inclined to violence - it is the very antithesis of his natural temperament. Pinchas is a man of peace, who did what he did with the sole aim of “turning away My wrath from the children of Israel.”
This also explains the significance of another statement by Rashi. After emphasizing that Pinchas was Aaron's grandson, the Torah writes “And the name of the smitten Israelite, who was smitten with the Midianite, was Zimri the son of Salu, a tribal prince of the Shimonites.” On which Rashi comments, “On the same occasion that the righteous one's lineage was cited in praise, the wicked one's lineage was cited in detriment.” But what “detriment” is there in Zimri's being a Shimonite prince?
Those who looked with a negative eye on Pinchas' motives, saw his “cruelty” even more strongly underscored when contrasted with the motives of the man he killed. Pinchas slew a man, while that man was engaged in an act of “love”; Pinchas was giving vent to his own violent passions, while Zimri acted out of a selfless concern for his constituents, putting his own life on the line (for surely he knew that some “zealot” might take it upon himself to kill him) to save his tribe through his bold attempt to legitimize their sins. If Pinchas did the right thing, he did it for all the wrong reasons, while Zimri might have done a wrong thing, but was motivated by an altruistic love for his people.
G-d, who knows the heart of every man, spoke to dispel this distorted picture. Pinchas, He attested, inherited the peace-loving nature of his grandfather, while Zimri was every inch a descendent of Shimon, whom Jacob rebuked for his heated and violent nature.
Indeed, the Talmud describes a hypocrite as “one who does the deeds of Zimri, and asks to be rewarded like Pinchas.” Zimri's “kindness” was the ultimate hypocrisy: instead of fulfilling his role as the leader of his people by prevailing upon them to cease the behavior that was destroying them, he pursued the fulfillment of his own passions, without regard to the terrible consequences to their spiritual and physical well-being - all the while disguising his act as selfless and self-sacrificial. In contrast, Pinchas' deed was “hypocritical” in the positive sense: ostensibly violent and cruel, but in truth a selfless act of peace.
This essay is based on a talk by the Lubavitcher Rebbe, Shabbat Pinchas 5725 (July 24, 1965) and on other occasions (9).
 Numbers, ch. 25; Rashi, ibid; Talmud, Sanhedrin 81b-82b and 106a.
 Talmud, Sanhedrin 82b, and Midrash Rabbah, Bamidbar 21:3, as quoted by Rashi on verse.
 Midrash Tanchuma, Yitro 7.
 Thus, the law that “zealots smite him” falls under the unique legal category of “a law that is not instructed”: if a would-be zealot comes to the court and inquires if he is permitted to “smite” a transgressor, he is not given license to do so (Mishneh Torah, Laws of Forbidden Relations, 12:5). Indeed, the very fact that he has come to ask disqualifies him---someone who needs to ensure, in advance, that he is backed by the court, is no zealot. The true zealot has no thought for himself: not of his feelings on the matter, not of his personal safety, not even of the moral and spiritual implications of his act on his own self---he doesn't even care if what he is doing is “legal” of not. He is simply determined to put an end to a situation that incurs the divine wrath against Israel(see Likkutei Sichot, vol. II pp. 344-346).
 Midrash Rabbah, Bamidbar 20:25.
 Ethics of the Fathers, 1:11. Again, what is relevant is not Aaron's spiritual station as Kohen Gadol, or “high priest” to G-d (in contrast to Jethro, who served as priest to idols), but Aaron's nature and personality. This explains why Rashi, in quoting the Talmud and Midrash, omits the reference to Aaron as “the Kohen,” despite the fact that it appears in the verse and all Rashi's sources (!)
 Genesis 49:5.
 Talmud, Sotah 22b.
 Likkutei Sichot, vol. XII pp. 160-170.