The Torah opens Parshas Pinchas by telling us Pinchas’ lineage: Pinchas ben Elazar ben Aharon haKohen. Rashi explains that Pinchas was being belittled for being the mere grandson of an oveid avodah zarah (Yisro) who dared kill a Nasi; therefore, the Torah stresses his connection to Aharon.
What good did it do for the Torah to repeat who Pinchas’ father and paternal grandfather was? Surely everyone already knew this information and it did not stop the criticism!
R’ Meir Shapiro, the Lubliner Rav, explains as follows: There are people who do certain mitzvos because it is in their nature. A person may be by temperament inclined to help others; he/she will gravitate towards doing chessed. Another person may have an intellectual streak and gravitate toward learning. The reward these people receive for their mitzvos does not compare to the reward of those who must fight against their nature to do the same actions, e.g. the weak student who spends hours pouring over a gemara; the person who in introverted or who prefers to keep to himself who goes out of his way to help others.
The shevatim knew that what Pinchas had done was right, but they downplayed his accomplishment. “It was in his nature,” they said, because what else would you expect from the grandson of an oveid avodah zarah. The Torah therefore tells us that Pinchas' character was akin to his grandfather Aharon haKohen, the great rodef shalom. He had to overcome his natural instincts to act with zealotry, and he therefore deserved the great reward Hashem bestowed upon him.