Pinchas the son of Elazar the son of Aaron (25:11)

Why does G‑d refer to Pinchas as the "son of Elazar the son of Aaron"? Because the tribes of Israel were mocking him, saying "have you seen this 'son of the fattener', whose mother's father (Jethro) fattened calves for idolatrous sacrifices, and now he goes and kills a prince in Israel?" Therefore, G‑d traces his lineage to Aaron.

- Talmud, Sanhedrin 82b

Few professions are as cruel and inhumane as the fattening of calves for slaughter. So when Pinchas slew Zimri, many said: "Look at this holy zealot! He acts as if motivated by this burning desire to avenge the honor of G‑d but, in truth, he has merely found a 'holy' outlet for his cruel and violent nature. After all, it's in his blood - just look at his maternal grandfather…" So G‑d described him as "Pinchas the son of Elazar the son of Aaron" in order to attest that in character and temperament he actually took after his paternal grandfather - the compassionate and peace-loving Aaron.1

The true greatness of Pinchas lay in that he acted in blatant contrast to his inborn instincts, conquering his nature to bring peace between G‑d and Israel.

- Rabbi Menachem M. Schneerson, the Lubavitcher Rebbe, of righteous memory

The founder of Chabad chassidism, Rabbi Schneur Zalman of Liadi, once declared: "The entire point of chassidism is that man should transform the nature of his character."

A chassid once complained to Rabbi Schneur Zalman's grandson, Rabbi Menachem Mendel of Lubavitch: "Rebbe, I don't know what to do. I lack all desire and pleasure in the study of Torah."

"That's fantastic!" replied Rabbi Menachem Mendel. "Indeed, you are a lucky man. But what am I to do about the fact that I so strongly desire to learn…?!"