Rav Zalman Sorotzkin in his Oznayaim laTorah writes (based on Chazal) that when Pinchas killed Zimri, the tribe of Shimon was ready to pounce on Pinchas and attack him. A miracle happened -- Pinchas’s neshoma departed and he appeared dead, thus escaping their wrath. R' Sorotzkin quotes that mekubalim teach that when revived, rather than Pinchas' own neshoma returning to his body, the neshomos of Nadav and Avihu descended instead.
Why davka the neshomos of Nadav and Avihu? What do they have to do with the story? Obviously when dealing with sisrei Torah there are limits to what we can understand, but R’ Sorotzkin reveals at least some of the surface meaning. The sin of Nadav and Avihu was that they were “moreh halacha bifnei rabam.” Rav Sorotzkin explains that Nadav and Avihu desired an even greater hisgalus of Hashem than could be provided by their rebbe, Moshe Rabeinu. The gemara (Brachos 7) relates that Hashem wanted to reveal everything to Moshe when He appeared at the burning bush. Moshe, however, turned away; he felt he was not yet ready for such a revelation. Later, at mattan Torah, Moshe regretted his decision and begged Hashem, “Hodi’eini na es derachecha.” Hashem responded, “When I was ready you refused; now that you are ready I am not.” Nadav and Avihu understood that was a limit even to what Moshe apprehended – Moshe forever suffered the shortcoming of having turned away at the sneh. That level of full understanding was what they aspired to. That’s why they tried to offer ktores in the kodesh kodashim even without instruction or advice from their rebbe, Moshe. In that respect, they jumped beyond what they were ready for.
Remember the end of last week’s parsha: Zimri approached Moshe with Kozbi and challenged Moshe to stop him. Moshe froze – “nisalma mi’menu halacha.” It was Pinchas who reminded Moshe of the din of “kana’im pog’im bo,” and who carried out that halacha. Pinchas at that moment had the clarity to see and act when even Moshe Rabeinu himself was unable to. Nadav and Avihu’s desire to surpass their rebbe may have gone awry in their lifetime, but the spirit of their actions, the ability to transcend even Moshe Rabeinu, was fulfilled a generation later through the actions of Pinchas.
(Side point: Rashi in our parsha writes that Moshe’s not knowing the halacha of yerusha when asked by the Bnos Tzelofchad was a punishment for his declaring that all difficult shaylos should be brought to him. Why does Rashi not bring up this punishment at the end of P’ Balak where we learn that Moshe forgot the din of “kana’im pogim bo”? In both cases Rashi uses the term, "nisalma mi'menu halacha.")