"They put fire in them [the censers] and placed incense upon it." (Lev. 10:1)

A Kabbalistic approach: We can gain a clearer picture of the sin of the sons of Aaron when noting that the Torah wrote, "they placed upon it" instead of "they placed on them" [there had been two censers, each brother having entered with his own censer]. When you compare what the Torah writes in connection with the 250 men who offered incense (Num. 16:18) you will find that the fire and incense is described with the words "they placed it on them [plural]". When the instruction as to the correct procedure is issued in the Torah, Moses had told these men, "Place on them incense." (Num. 16:17) (No mention had been made of fire although the 250 men added their own fire.) The word for "on it": "aleyha", as distinct from "on them": "aleyhem", is an allusion to the attribute of Judgment.

The incense is meant to draw down an abundance of heavenly blessings….

The Torah is trying to give us an insight into the thinking of Nadab and Abihu at that time. They knew that incense was intended to counter, indeed to stop, the attribute of Judgment in its tracks, as we know from Moses in the verse: "they place incense [in Hebrew, 'ketoret'] to placate Your anger." (Deut. 33:10) The word "ketoret" itself means "establishing a spiritual affinity", as illustrated by the Aramaic translation of the word "tied" [in the phrase "the midwife took a crimson thread and tied it on his hand…" (Gen. 38:28)] - "vekataret".

The spiritual affinity established by means of the incense is meant to draw down an abundance of heavenly blessings by means of the attribute of Judgment, which in turn will confer these blessings on the person burning up the incense.

The sin of the person offering incense with such considerations consists in the fact that it is not permissible to direct one's offering to any other attribute of G‑d than that of the name Havayah. Seeing that Nadab and Abihu erred in the address to which they offered their incense, we do not find it described as "as a fire offering of pleasing fragrance to G‑d", but rather the very attribute of Judgment to whom they addressed their offering smote them. This is the meaning of the words "…fire came forth from the presence of G‑d and consumed them so that they died".

[Selected with permission from the seven-volume English edition of "The Torah Commentary of Rebbeinu Bachya" by Eliyahu Munk.]