Lévitique 10:2

Rabbi Eliezer said, the sons of Aaron died because they rendered a halachic decision in the presence of Moshe their teacher. Rabbi Yishmael said they entered the Temple while intoxicated, and the proof is that after their death the Torah warns against performing the service in the Temple while drunk.

And a flame of fire came out from before the Lord (as) with anger, and divided itself into four streams, (or lines,) and penetrated their nostrils, and burned their lives (souls) without destroying their bodies; and they died before the Lord.

they died before God thinking they were doing something acceptable to God.

AND THERE CAME FORTH FIRE ‘MILIPHNEI’ (FROM BEFORE) THE ETERNAL. The meaning of “miliphnei the Eternal” is like: “from before the pnei (presence of) the Eternal.” The student learned [in the mystic lore of the Cabala] will understand, for I have already explained this. The nature of the sin of Nadab and Abihu you may know from that which it says, and they offered ‘strange fire’ before the Eternal, and it does not say “and they offered before the Eternal ‘incense’ which He had not commanded them.” Thus they put incense upon the fire, similarly to that which Scripture says, they shall put incense ‘b’apecha’ (before Thee), and they directed their thoughts only to this, (literally: ‘before Thy anger’). It is for this reason that the burning of incense stays the plague. (Numbers 17:11-15). However, the priest who offered it had to direct his mind to the perfect Unity of G-d. And since Nadab and Abihu directed it only to the attribute of justice (see Note 58), therefore they were punished.” See further, my Hebrew commentary p. 46. and thus it was not a fire-offering of a sweet savor. This is the sense of the expression, and they laid ‘aleha’ (upon her) incense. (upon her) is an allusion to the attribute of justice. and it does not say “and they laid upon them [i.e., the censers] incense,” as is said with reference to the company of Korah, and put fire therein, and put incense upon them, and again it is said there, and put incense upon them., Verse 17. But here it says upon her, to allude [to the fact] that they put the incense only on “the fire,” [and therefore it says], And there came forth fire from before the Eternal, and devoured them. It is possible that the verse which He said, Ye shall offer no strange incense thereon, alludes to this, meaning that he should not cause it [the fire] to be strange. It is in connection with this that Scripture states, when they [Nadab and Abihu] drew near before the Eternal, and died, which means “when they offered l’phanav they died.” alone [to the attribute of justice alone, as explained above] they died.

ותצא אש מלפני ה' ותאכל אותם, this is a reference to the fire mentioned in 9,24 The construction is similar to the one in Exodus 19,8 which we explained at the time, as well as the verse in Judges 17,3-4 where the fact that Michah returned the money to his mother is also mentioned twice in successive verses. In neither of these verses does the author refer to an additional event. Here too, the two verses refer to the same fire.

At the time the heavenly fire emanated in order to consume the sacrificial meat on the altar in the courtyard of the Tabernacle, it consumed the two sons of Aaron on its way. This fire had meant to consume only the incense, but seeing that the sons of Aaron had been in its way it consumed them also and they died as a result. This is the meaning of

'וימותו לפני ה, “they died in the section of the Tabernacle described as ‘in the presence of the Lord.’” As soon as the news reached Aaron he wanted to interrupt performing the service in order to mourn his sons;

'ותצא אש מלפני ה, “and fire came forth from before the Lord, etc.” this was because they had not acted appropriately. Even though it is in order to use man made fire, as we have read specifically in Leviticus 1,7: ונתנו בני אהרן אש על המזבח, “the sons of Aaron are to put fire on the altar, this was in addition to the heavenly fire which had come down a single time. Before G–d had demonstrated acceptance of the Israelites’ offerings in the Temple precincts, they were not allowed to use man made fire, as it would have belittled the miracle G–d provided. People would have argued that their offerings had been consumed by man made fire. This is why G–d struck these two sons of Aaron with fire emanating in heaven. Our author has difficulty with this interpretation, as the Torah had written in Leviticus 9,24: ותצא אש מלפני ה' ותאכל על המזבח, “fire came forth from before the Lord and consumed the burnt offering and its fat parts.” How could the sons of Aaron therefore have belittled G–d’s fire by what they did? This is why the sages in the Talmud tractate Yuma folio 53, state that the sons of Aaron did not die because of this, but because they had dared to usurp the authority of Moses and Aaron by making halachic decisions without first having consulted with these leaders.