There are various reasons given by the Sages, according to what is recorded in Torah, as to the nature of the transgression of Nadav and Avihu, two of Aaron’s four sons, and their subsequent demise, inasmuch that they were consumed by eish (fire) sent by H’Shem from Shomayim (Heaven). However we learn of a previously unmentioned one, at the beginning of Acharei:
And H’Shem spoke unto Moses, after the death of the two sons of Aaron, when they drew near before H’Shem, and died; and H’Shem said unto Moses: “’Speak unto Aaron thy brother, that he come not at all times into the holy place within the veil, before the ark-cover which is upon the ark; that he die not; for I appear in the cloud upon the ark-cover’” (Leviticus 16:1-2, JPS 1917 Tanach). Here the Torah infers another reason for the deaths of Nadav and Avihu.
Whereas previously, in parashas Shemini, it is written that they approached the Mishkan (Sanctuary). The Torah infers that they approached H’Shem’s presence that appears between the two golden cherubim on the kapores – the cover of the Ark. When Aaron is cautioned in this passage, not to enter the Kadosh Kadoshim at all times (only once-a-year on Yom Kippur) this implies that Nadav and Avihu entered the Kadosh Kadoshim, behind the paroches, the veil or curtain that separated the inner sanctuary where the ark of the covenant was kept, from the rest of the Mishkan. This teaches us of the significance of yiras H’Shem – proper awe, reverence and respect towards H’Shem Elokim (The L-rd our G-d).
“Sanctify yourselves therefore, and be ye holy; for I am H’Shem your G-d.”
– Leviticus 20:7, JPS 1917 Tanach