Pinchas was given great rewards for his act of zealotry. Officially, he was not yet a Kohein as he was not anointed with the שמן המשחה, anointing oil, with Aharon’s sons.
Hashem blessed Pinchas with ברית כהונת עולם, a covenant of eternal priesthood. The Ibn Ezra says that all future High Priests, כהנים גדולים, were descended from Pinchas.
Rabbeinu Bechaye adds that when one is שופך דמים של רשע, spills the blood of the evil, it’s as if he has offered a קרבן, sacrifice.
At first, the people were uncertain how to react to what Pinchas had done. It was only when Hashem clarified matters, did it become clear the greatness of the heroic act of Pinchas. Shabbat Shalom