Rabbi Naftali Tzvi Yehuda Berlin, aka "the Netziv" (1817-1893), discusses this theme in his commentary to last week's parasha, parshat Pinchas (Numbers 25:10-30:1). Some background: Pinchas, the grandson of Aaron, had (in the previous parashat) seen a fellow Jew, Zimri, openly fornicate with a Moabite priestess in desecration of G-d's Name. Zealously, Pinchas took a spear and in one thrust impaled the couple in the act. G-d rewards Pinchas for his zealotry with the Covenant of Eternal Priesthood (previously Pinchas was only a Levi for reasons I can go into if you like). Pinchas was also rewarded with the Covenant of Peace. Numbers 25:12. Commenting on this verse, the Netziv says that under normal circumstances, when a person kills another human being, that makes an indelible impression upon him; he is changed forever. The blessing of peace that Pinchas received for his exercise in violent zealotry to protect the honor of G-d was so that this murder would not have the normal effect upon the soul of Pinchas that it would on others.

The Netziv comments that zealotry is very, very dangerous. In some cases, it can be the most appropriate medicine and other times it can backfire. The Netziv comments to the verses in Deut. 13:13-19, regarding the Ir HaNidachat -- a city that was to be destroyed because the majority of its inhabitants became idol worshipers -- that G-d must state there that if we follow that commandment, G-d will "give you mercy and be merciful to you and multiply you, as He swore to your forefathers." This is the same type of promise that G-d gave to Pinchas. But contrast this with the story of Eliyahu (Elijah), who seized the prophets of Baal and killed them all at the Kishon Brook (1 Kings 18:40). Queen Jezebel heard this and plots revenge, forcing Eliyahu into hiding. At his cave, G-d confronts Eliyahu and asks him why he is there. He responds: "I have acted with great zeal for Hashem, G-d of Legions, for the Children of Israel have forsaken Your Covenant; they have razed Your altars and have killed Your prophets by the sword, so thatI alone have remained, and they now seek to take my life." 1 Kings 19:10. G-d sends him to stand on the mountain, where G-d confronts him with wind and earthquakes and fire and again asks Eliyahu why he is there, and he responds as he did in verse 10. The Midrash explains this difficult story, noting that G-d's questions to Eliyahu imply some measure of criticism. G-d chastised Eliyahu for not talking properly about His people. "Do not say about My People 'they have not kept Your Covenant! Do not talk that way about Jews! You should have said, "They are Your children, descendants of Abraham, Isaac and Jacob." But when Eliyahu didn't understand the criticism, he sent him to the mountain and asks again, but Eliyahu still responds that he was zealous for G-d's sake. G-d tells Eliyahu "You were zealous at Shittim. Here you are zealous and there you are zealous -- always acting with zealotry!"

According to Rav Mordechai Katz, zt'l, the distinguishing factor of the zealotry of Pinchas and Eliyahu, is that while both acted nobly, Pinchas only defended G-d's honor without defaming the Jewish people, while Eliyahu did both. Our sages tell us that Eliyahu lives today, atoning for this slander, by attending ever brit milah and every Pesach table so he can see first hand that the Congregation of Israel does care about the commandments and observes them.

In Harry Potter, we learn that one can apperate -- travel magically from one place to another -- only through concentrated "Deliberation, Determination and Decisiveness," being careful to know exactly what you're doing, thinking and saying so that your entire body successfully makes it to where you want to go. The Netziv might tell us that Judaism demands the same kind of attitude, because without conscious and careful deliberation, determination and decisiveness, our careless thoughts or words might fail to deliver us to the exact place where we belong.