Speak to Aharon your brother. He shall not come at all times into the Sanctuary, within the Curtain, in front of the Cover that is upon the Aron, so that he shall not die.
Before there was death, there was still death. Had Man not sinned and eaten from the Tree of Knowledge, people would still have had to transition between life as we know it, and a higher form of consciousness. In that sense, death was part of life from the very beginning. That transition, however, would have been very different from what we know today as death. The original form of that transition was entirely positive. The leaving behind of concerns and occupations that are part of what we call life would only have strengthened Man, rather than weakened him. It would have equipped him for an enhanced, supercharged life.
Instead, Adam’s sin changed death into something negative. The transition now required negation and destruction of the previous life. Specifically, the sin attached itself to aspects of life that involve its continuity: eating, and reproducing. The new death brought cessation where there had previously been continuity.
Yitzchok had hoped to bring about a tikkun of eating through Esav. It is known that Esav had within him the power of remediation and change. That is why his neshamah is said to contain the souls of geirim. Yitzchok sought to harness this ability through the mitzvah of kibud av v’eim, of honoring parents. Yitzchok had Esav hunt and prepare food for him. Through this, Yitzchok sought to reach the potential geirim within Esav’s neshamah, and raise them up to where they would join the Jewish people. Yitzchok also thought that this process would begin the tikkun of eating, which had fallen so terribly though Adam’s sin. (Esav, however, assumed that one honored parents only with spiritual things. That is why he brought signs of piety to his father, asking how to take ma’aser from salt and straw. Yitzchok, however, insisted that one manifested the honor due to parents specifically through serving them materially, through elevating the physical. This is the idea behind the meals that he had Esav prepare for him.)
This is also the intent of the gemara that teaches that the nations of the world were skeptical and critical in listening to the first few of the Ten Commandments. The listeners cynically observed that those commandments seemingly functioned to enhance G-d’s honor. Once, however, they heard the commandment about honoring parents, they conceded that all the commandments were just. Their first reaction presupposed that this Torah that was being given to Man concerned itself only with spiritual matters. The non-Jews cynically found this approach wanting. How could such an approach remedy the change in Man and his entire physical world that had been brought about by Man’s sin? When they heard the commandment about honoring parents, and realized that it afforded Man an opportunity to take pedestrian matters like food and drink and turn them into kedushah, they recognized the awesome potential of Torah.
Yitzchok’s attempt at beginning the tikkun required a major tweak, which came through the insight of Rivka. She realized that Esav’s engagement of the physical world would only degrade it further, rather than elevate it. That elevation would have to come through the descendents of Yaakov.
While Yitzchok and Rivka addressed the tikkun of eating, the parallel tikkun of death waited for Nadav and Avihu. Their story is juxtaposed in our parshah to the avodah of Yom Kippur. In a word, we experience elevation each Yom Kippur through self-imposed privation and negation of our physical wants and desires. This is remarkably similar to the function of death, which also allows us to attain a higher consciousness, but only by eradicating life itself. (As we explained above, precisely that was the change brought about by Adam’s sin.)
Nadav and Avihu were determined to take control of this process, and enlarge it beyond the single day of the year model. By entering within with their fire, they hoped to achieve the same elevation that ordinarily was available only on Yom Kippur, when the Kohen Gadol entered the Kodesh Kodashim. They did not concern themselves with the fact that they would forfeit their lives thereby; they wanted the elevation available in the transition to a higher state, regardless of the price they would have to that they were after.
The Torah, however, took a dim view to this approach. “He shall not come at all times into the Sanctuary… so that he shall not die.” The Torah does not want us to achieve those spiritual heights by negating our physical existence. This is the point of Yom Kippur – that we reach those heights and live to cherish them. This is available to us only on Yom Kippur.