The rabbis who comment on Torah are of two different minds about this incident. One camp, the more mundane camp, see this as Divine punishment, and scramble to find an offense. Thus the Talmud cites Exodus 19: 22: "And let the priests also, which come near to the LORD, sanctify themselves, lest the LORD break forth upon them." The next commandment listed in the Torah after this incident is "Do not drink wine nor strong drink, thou, nor thy sons with thee, when ye go into the tabernacle of the congregation, lest ye die: it shall be a statute for ever throughout your generations." [Leviticus 10: 9], causing some rabbis to speculate that Nadab and Abihu were drunk, and are struck dead for being drunk inside the Tabernacle. There's quite a lot of speculation as to what offense the two boys committed that caused such a swift and immediate punishment, none of which strikes me as convincing.
Others regard this incident in a more esoteric way, claiming that Nadab and Abihu either consciously sacrificed themselves to God to consecrate the Tabernacle, or that God brought them too close, and they were unable to withstand the spiritual intensity of that peak moment. Rashi cites the following: "...at the door of the tabernacle of the congregation before the LORD: where I will meet you, to speak there unto thee. There I will meet with the children of Israel, and the tabernacle shall be sanctified by my glory." [Exodus 29: 42-3].
My own understanding of this tragedy is that God, through Moses and Aaron, was implementing a brand-new spiritual technology, and like any new technology, its safety margins were not well understood. The Tabernacle was the devotional focal point of an entire nation of millions of people. The intensity of that focus was sufficient to incinerate a person under that lens who did not have sufficient protection. It was similar to touching the third rail during the dedication of a new subway.
Moses' comment seems to suggest that Nadab and Abihu were sanctified rather than punished, and his comment silences Aaron, who just watched his two eldest sons die. Moses summons two of Aaron's cousins to drag away the corpses of the two boys (touching a corpse would desanctify Aaron), and Moses warns Aaron and his surviving sons that if they mourned the death of the boys in the usual manner customary to Israelite custom, they would also be struck dead. Instead, the entire community of the Children of Israel are to mourn the death of the two boys for the immediate family.
In their shock, Eleazar and Ithamar burn the sacrifice that should they should have eaten, and Moses is very angry with them. Aaron responds to Moses: "Behold, this day have they offered their sin offering and their burnt offering before the LORD; and such things have befallen me: and if I had eaten the sin offering to day, should it have been accepted in the sight of the LORD? " [Leviticus 10: 19]. This is the closest that Aaron comes to complaining about or mourning the death of his sons. There is so much grief and pain in that line that it melts the heart. We get a sense of the agony with which Aaron would have internalized the metaphor, ingesting the meat of the roasted sacrificial lamb while his own lambs were roasted and sacrificed.
This is sufficient to make Moses back off: "And when Moses heard that, he was content." [Leviticus 10: 20].
The Torah portion ends with a discussion of kosher dietary laws. The animals that are regarded as unclean are mentioned, and prohibited. One who eats unclean animals makes himself "abominable" in the sight of the Lord [Leviticus 11: 43]. The purpose of such restrictions is as follows:
For I am the LORD your God: ye shall therefore sanctify yourselves, and ye shall be holy; for I am holy: neither shall ye defile yourselves with any manner of creeping thing that creepeth upon the earth. For I am the LORD that bringeth you up out of the land of Egypt, to be your God: ye shall therefore be holy, for I am holy. [Leviticus 11: 44-5].This strikes me as very curious and unique. God wants us to be holy, as He is holy. What does that kind of holiness mean? Other religions have buildings, books, artifacts that are holy, but here, God wants people to be holy. We sanctify the holiness of God by being holy ourselves.
As Masons, we are engaged in building that spiritual building, that house not made with hands, eternal in the Heavens. That building is holy, and the Deity to whom it is directed is holy, so we have to be holy in order to build it. Masons understand that the Temple of Solomon, so stately and magnificent, could not withstand the ravages of barbarous force. Freemasonry notwithstanding still survives. It survives because we are building the new Temple not in a place where fire, ram, wrecking ball, or bomb can harm it. We are building it out of our immortal souls, as living stones, which have to be holy if they are to make a holy structure.