Rashi on Numbers 25:14/רש"י על במדבר כ״ה:י״ד

1ושם איש ישראל וגו׳ AND THE NAME OF THE ISRAELITE [WHO WAS SMITTEN] — Wherever it gives the genealogy of a good man in praise of him, it gives the genealogy of a bad man mentioned in the same story to disparage him (Midrash Tanchuma, Pinchas 2).

2נשיא בית אב לשמעני A PRINCE OF A FATHER’S HOUSE AMONGST THE SIMEONITES — of one of the five 'father’s houses' which were of the tribe of Simeon. — Another explanation: This is stated to tell the praiseworthiness of Phineas: that although this man was a prince he did not refrain from showing his zeal against the profanation of the Divine Name — on this account Scripture tells you who it was (a prince!) that was smitten by him.