Nadab and Abihu, the two sons of Aaron, were burned when making the incense offering in a zealous yet inappropriate way. Their souls were lost or not at peace, until the act of Pinchas. He slew Zimri and Cozbi in a zealous yet appropriate way. The spiritual power generated by his standing in anticipation of this brave deed, led to the lost souls of Nadab and Abihu affixing themselves to his Nefesh. Their souls were then rectified through his deed, and it was as though Aaron had received his two lost sons back to life.
Tzelofchad had not had any sons and his five daughters argued that they, too, deserved a portion of the Land, even though land had been only apportioned explicitly to sons. They personified the five states of gevura; therefore, Moses was unsure whether or not they deserved a portion in the Land, as the earth, and specifically the Land of Israel, personifies Malchut. G-d told him that since even the iterations of the five states of gevura that do not themselves enter Malchut shine some of their light into Malchut, they deserve a portion of the Land.
At the time of his momentous act of zealotry, Pinchas saw his own name (which has a numerical value of 208) rise heavenwards, and realized that his name had the same numerical value as that of Isaac the patriarch, who personified the attribute of Justice on earth. He also saw the letter mem fly in the air. This letter is an allusion to death - in Hebrew, "mavet" and this inspired him to be a barrier of "death". The Zohar, commenting on the lance which Pinchas used to slay Zimri, describes Pinchas as "garbing" himself with this lance in order to mete out justice here on earth so that mercy could be invoked in the parallel regions of Heaven.
The main principle in serving G-d is to attach oneself to the inner essence and vitality of the attributes. The trait of chesed is expansive, and gives to everyone; it is the essence of love. However, a person attached to the externality of chesed may not be able to distinguish between one type of love and another, and may love everyone - the good and the evil, G-d forbid. The main thing, however, is to be attached to chesed's inner nature. For then, strictness is part of chesed and this is the inclusion of the gevura in chesed. Gevura restrains overabundant chesed, and chesed uses gevura for compassionate purposes; when there is inclusiveness among the sefirot, then each one contains and tempers all the others.
The sons of Korach helped plot the rebellion, but during the rebellion they entertained thoughts of repentance. Therefore, a perch was fortified for them in Purgatory and they dwelt there; they did not perish along with everyone else. However, they only thought about repentance and did not actually repent; in fact, to all outward appearances, they remained rebels to the very end. Therefore, their punishment fit their offense: to all outward appearances, they perished along with everyone else, but hidden from view in the interior of the earth, they remained alive.
Because Pinchas avenged G-d's honor, He did not eradicate the Jewish people. Pinchas was rewarded in that he and his progeny were conferred with the status of priesthood. The Shelah reminds us that when we see an act that desecrates the name of G-d or the Torah, we should be zealous to stop it.
After the sin of the Golden Calf, Moses also turned away G-d's wrath and ended the plague, yet he wasn't rewarded with the priesthood or with something that could be bequeathed to his descendants, since Moses used his power of prayer to nullify the negative decree from Above. We see that the effect on the Jews was minimal - i.e. they sinned again. Therefore, the reward Moses received was also only temporal. Pinchas, on the other hand, affected the world from below, by causing the Jews to repent through his deed. Accordingly, G-d rewarded him in a similar fashion, with an eternal change of status as a priest.
"Go!" replied the tailor, "but continue to wear this overcoat."
Shabbat Shalom.