Chazal say that the sin for which Nadav and Avihu were killed was that they looked at Moshe and Aharon and said "Soon these elders (Moshe and Aharon) will die and we will take their place as the leaders of the nation."  Along the same lines, another explanation is that they failed to consult Moshe before performing their service.  It would seem therefore that they were punished for there arrogance and narcissism, and that the specific object of the "Aish Zara" had only incendental significance.  However, the Tora itself focuses specifically on the Korban, which requires explanation.In his essay "Lonely Man of Faith," Rav Soleveitchik describes human nature in a dialectical manner, based on the two accounts of man's creation in Bereishis.  The first Adam is given the command "Lachbosh et Haaretz," representing  "majestic man," who is driven to conquer the world and to strengthen his own ego.  Though he is a social animal, his interactions with others, even his loved ones, are artificial and are done in a competitive manner solely for selfish purposes.  So too, his relationship with G-d is based in narcissim. This explains why Adam the first's wife was created along with him, requiring no sacrifice.Contrarily, Adam the second is defined by "Da lifnei mi atah omed."  If one realizes he is standing before an omnipotent being, he will nullify his own will and take on the will of his creator.  Adam the second's Chava required him to be made unconcious, and then to sacrifice his rib.  In other words, he had to nullify his will and sacrifice of himself, which created a relationship based on truth, termed "Ezer K'negdo." There are times when man must operate as Adam the First, fighting and competing "Lachbosh et Haaretz."  But when bringing a Korban, when man is ment to feel as if the goat or bird is in place of himself, and that he should be the one on the altar, da lifnei mi ata omed must take over.  The act of a Korban requires a nullification of the self, and as such, a Korban used as an expression of narcissim would be an abomination. This concept is illustrated harshly in the haftorah for Parshat Mishpatim, where we are told that when the Jews took slaves unlawfully, the ultimate selfish act, they had essentialy broken their covenant with G-d and sealed their fate of exile.  The Point:  Unless man is willing to give of himself to others, he has by definition precluded having any true relationships, whether with man or G-d.