Pinchas' gift

B"H

Bamidbar (Numbers) Ch. 25:11,13 reads:
פינחס בן אלעזר בן אהרן הכהן השיב את חמתי מעל בני ישראל בקנאו את קנאתי בתוכם ולא כליתי את בני ישראל בקנאתי
והיתה לו ולזרעו אחריו ברית כהנת עולם

Loosely translated:
Pinchas son of El'azar son of Aaron the Kohen (i.e., priest) has turned away my wrath from the Jewish people in his act of zealousness for Me, and I shall therefore not annihilate the Jewish people in my jealousy... And it shall be to him an eternal covenant of priesthood.

Two questions pop out immediately:
1) Why does the Torah find it necessary to list the entire lineage of Pinchas back to Aaron the Kohen.
2) Why was to covenant of priesthood bestowed upon Pinchas, for priesthood is not something that's transferable, purchasable or giftable -- it's can only be hereditaraly given from father to son. Nowhere else in scripture such an unusual gift can be found.

An explanation, but first some background:
Rashi on 25:11 writes:
The tribes were disapproving of Pinchas' actions saying: have you seen the son of Puti, whose mother's father has fattened calves for idol-worship. This person (=Pinchas) has murdered an prince of a tribe of the Jewish nation. Therefore scripture has traces his linage to Aaron the High-Priest.

Why did Rashi uses the long winded "whose mother's father fattened calves for idol-worship" where it could more simply stated "whose grandfather fattened..."?

Yitro (Pinchas' maternal grandfather) was the high-priest of Midian. In his capacity as high-priest he would take calves and feed them to satiation but he wouldn't stop there. After the calves were satiated he continued to stuff them with food until they could hardly function, and then he stuffed them with more food still. The sole purpose of this exercise was to really fatten the calves. But for what purpose, you might ask... Answer: for the sole purpose of bringing a delicious, fat dribbling, juicy sacrifice to idols.

Essentially, Yitro was performing acts of cruelty to animals just for the loathsome activity of idol-worship. Indeed he embodied the attribute of cruelty. (This, of course, was before he turned away from idol-worship and in fact converted to the Jewish belief.)

There's a known principle (need citation) that a father's character traits are inherited by his daughters, while a mother's character traits are inherited by her sons.

So, in saying "whose mother's father fattened calves for idol-worship", the tribes were claiming that Yitro has given his character trait of cruelty to his daughter and she has given it, in turn, to Pinchas. Indeed, with this line of thought, they reasoned that Pinchas has killed Zimri, the prince of the tribe of Shimon, not out of zealousness to G-d, but rather out of cruelty!!! They were prepared to kill him for that....

To settle the matter, the Torah writes "Pinchas, son of El'azar, son of Aaron the Koeh". This is to say, don't look at Pinchas' pedigree on his mother's side, but rather on his father's side. Who's his father? El'azar son of Aharon the Kohen. About Aharon the Kohen it is stated in Pirkei Avot (Ethics of the Fathers) Ch. 1:12:
אהרון - אוהב שלום ורודף שלום אוהב את הברייות ומקרבן לתורה

Loosely transslated:
Aaron (the Kohen) -- loves peace, chases after peace, loves all creatures and draws them closer to the creator.

So, we find that Aaron's attribute is that of love. Since Aaron is the prototype and first Kohen, we must conclude that this attribute of love is inherited by all priests, since priesthood is modeled after Aaron.

So, the Torah is telling us: one might conclude that Pinchas killed Zimri out of cruelty, but know this: Pinchas has done so only out of love for the Jewish nation, in order to stop the plague that was ravaging the Jewish nation at the time -- his turned away G-d wrath.

This then explains why priesthood was gifted to Pinchas: he always had that attribute of love for the Jewish people, though up to this point it was buried and concealed within him -- it did not manifest in the open. But now, that he has demonstrated his love for the Jewish nation, in the most grandiose way, he essentially taken that attribute of love and pulled it out from obscurity and into plain sight. The most appropriate gift to give him, then, is to officially brand him a Kohen -- "loves peace, chases after pease, loves all creatures...."