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 the desire to avenge the honor of G-d and save the people, 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.
The true greatness of Pinchas lay in that he acted in complete opposition to his nature, transcending his inborn instincts to bring peace between G-d and Israel.
(The Lubavitcher Rebbe)