And Pinchas the son of Elazar, the son of Aharon the Kohen, saw, and he rose from the midst of the congregation, and took a spear in his hand.
Torah Temimah: Sanhedrin 82a — what did he see? (Rav / Shmuel / R. Yitzchak).
English: What did he see? Rav said: He saw the deed and remembered the law. He said to Moshe: Brother of my father’s father, is it not thus that you taught us when you came down from Mount Sinai — one who cohabits with an Aramean woman, zealots strike him? He said to him: He who reads the letter, let him be the agent.
Recovery of a Sinai ruling that licenses zealotry in this case.
Pinchas acts only after recalling the law and receiving Moshe’s charge — zeal is yoked to Torah, not private passion.