Pinchas was acting according to halachah: Bo’el aramis kana’im pogin bo, “One who cohabits with a gentile, zealots may strike him.” If so, why is he referred to as a kanai, zealot? He was just doing what any other observant Jew would/should have done. Indeed, Chazal imply that Hashem chastised Moshe Rabbeinu for remaining passive during the moral outrage that took place. As a result, Moshe’s gravesite remains unknown to us. Chazal derive from here that one must be “bold as a leopard, as nimble as an eagle, as swift as a deer, and as mighty as a lion in executing the will of Hashem.” While this critique is only relative to Moshe’s extreme spiritual level, it does at least indicate that kana’im pogin bo is a halachah which applies to everyone.
Horav Mordechai Gifter, zl, explains that the punishment of death for a bo’el aramis may only be carried out by one who is a kanai. One who has achieved this elevated plateau of serving Hashem out of extreme love, only he may assume the mantle of executioner.
The Rosh Yeshivah explains that, unlike the misguided perception of many, a kanai is not an extremist. Any person who is willing to render his life for Hashem is not an extremist. He is an intelligent human being who has achieved a true level of greatness. He sees with an impeccable clarity what others do not see. His entire life revolves around carrying out the will of G-d. If one’s motives are not pristine, he ceases to be a kanai. He is a regular fellow, and, hence, he may not lay a finger on the perpetrator of this immoral act of sacrilege. To paraphrase Rav Gifter, “The kanai sees things from an altogether different perspective. His vision is clear and unambiguous. There is not an iota of deviation in his perspective. Thus, he sees what others do not. The kanai sees a bo’el aramis for what it really is. Everybody else sees an act of promiscuity. He sees the underpinnings of Judaism being yanked from their moorings. Therefore, he may act – while others may just watch.”
