When Pinchas saw a prince from the tribe of Shimon publicly cavorting with a Midianite princess, he took the law into his own hands. Using his spear, Pinchas killed them both. God praised his act of zealotry, rewarding him with the priesthood.
“Pinchas, the son of Elazar, the son of Aaron the kohen, was the one who zealously took up My cause among the Israelites and turned My anger away from them.” (Num. 25:11)
Why does the Torah need to point out Pinchas’ lineage here? This is particularly puzzling considering that the Torah just identified Pinchas a few verses earlier (25:7).
The Midrash (Sanhedrin 82b) explains that the tribal leaders mocked Pinchas: ‘His maternal grandfather [Jethro] fattened up calves for idolatrous sacrifices — and he had the audacity to murder a prince of Israel!’ Therefore, the Torah publicized Pinchas’ lineage through his father’s side, Aaron the High Priest.
This Midrash requires clarification. Why was it so important to respond to these disparaging comments? Furthermore, what does it help if one of Pinchas’ grandfathers was the high priest - his other grandfather was still a reformed idolater!
Rav Kook explained that the Torah does not ordinarily approve of such acts of zealotry. They are sanctioned only if the zealot acted purely for the sake of Heaven.
Onlookers might have suspected that Pinchas harbored secondary motives. Perhaps he sought to demonstrate his faithfulness to Israel and its monotheistic faith, despite a grandfather who was a convert from paganism. Therefore, God testified that Pinchas acted as Aaron’s grandson. What qualities characterized Aaron? The Sages wrote: “Be a disciple of Aaron, loving peace and pursuing peace, loving all people and drawing them near to the Torah” (Avot 1:12). Aaron, legendary for seeking the path of peace and reconciliation, would not have been suspect of ulterior motives. Pinchas’ action, the Torah emphasizes, was worthy of his illustrious grandfather. He acted as befits the grandson of Aaron the High Priest, with selfless intentions and a pure heart.
(Sapphire from the Land of Israel. Adapted from Olat Re’iyah vol. I, p. 394)