Today's passage covers the ordination of Aaron and his sons, the beginning of their ministry, the death of Aaron's sons Nadab and Abihu, and the fallout from that event.
Imagine the scenario: you're a newly-ordained priest. The sacrifices are not yet cold from your ordination ceremony, and you decide, being the eager young priest that you are, to start your ministering right away. Specifically, you decide to start with something simple: incense. You and your brother, also a newly-ordained priest, take some incense and burn it for God. Unfortunately, you reached into the wrong incense jar and burned the wrong stuff.
You know you've done wrong. But there are a lot of rules, and you're new. Surely there's a trial period. You'll just burn a sin offering, switch the incense, and do it properly tomorrow. Right?
Wrong. Because for you, there are no tomorrows. Because you'll be dead. Because God killed you.
This is exactly the scenario that Aaron's sons, Nadab and Abihu, find themselves in at the beginning of Lev. 10. In fact, the episode takes up so little space in the text that it's worth quoting in full:
(1) Aaron's sons Nadab and Abihu took their censers, put fire in them and added incense; and they offered unauthorized fire before the LORD, contrary to his command. (2) So fire came out from the presence of the LORD and consumed them, and they died before the LORD. (Lev. 10:1-2, NIV)
Keep in mind that just the two chapters before, in Lev. 8, these men were ordained as priests, and in Lev. 9 they offered their first official sacrifices.
I can just imagine the conversation Nadab and Abihu, or at least their deceased spirits, must have had with God after this incident: "Lord, what is the meaning of this? Our father, Aaron, singlehandedly created a golden god to rival You (Ex. 32), and all he got was a slap on the wrist. We offer the wrong incense, and You kill us! What's going on? Have You never heard of a probation period?"
I can only imagine God would not have been particularly sympathetic to this line of reasoning.
Of course, it does raise the question, "what was God thinking?" Given the exceedingly small amount of text devoted to this episode, we don't know what Nadab and Abihu were thinking when they offered the false incense. Perhaps they were doing it maliciously, trying to deliberately break God's laws. Perhaps they were drunk "on duty," as suggested by the commandment later in the chapter that priests must never drink wine or other strong drinks when in the tabernacle, on pain of death. (Lev. 10:9-11) Perhaps the incense they offered was poisonous, or extremely flammable, or otherwise dangerous to themselves or the tabernacle. God may have been perfectly justified in thinking they two brothers were up to no good.
On the other hand, we are now faced with an interesting double-standard. Aaron did indeed create the golden calf, but this was before he was ordained as high priest. Perhaps he was given a certain amount of leeway, since he was not officially the high priest at that time. The argument seems a bit tenuous to me, but it can be made regardless. Ordained priests are held to higher standards than the rest of the community. Moses is held to an even higher standard than Nadab and Abihu: when God commanded him to speak to a rock in order to have it pour forth water, Moses struck it with his staff instead. Water did flow out, but God became infuriated with Moses for disobeying him and declared that because of this insubordination, Moses would never enter the holy land. (Num. 20:1-13; Deut. 32:48-52) Moses slipped once, and indeed it seems only natural that it was bound to happen, given the whining of the Israelites, and God denied him the holy land.
Similarly with Nadab and Abihu: they were God's representatives within the Israelite nation. They of all people were supposed to uphold God's laws, no matter how minute. If they could not be trusted to light the proper incense, they may have gone on to more severe infractions.
Returning briefly to the land of the living, we learn that Aaron and his two remaining sons, Ezeazar and Ithamar, were not allowed to mourn for their dead relatives. They were specifically commanded not to uncover their heads or tear their clothes, traditional signs of mourning. (Lev. 10:6) The rest of the Israelites were allowed to mourn, but not the immediate family. (Lev. 10:7) Why? Perhaps because Aaron and his sons were supposed to know better. Maybe they could have warned their brothers against the false incense, and thus some of the blame fell upon them. Furthermore, they were supposed to reflect God's decision: if he killed Nadab and Abihu because of their infraction, Aaron and his other sons were supposed to stand by and accept the decision stoically.
I can only imagine, however, that this was a severe wake-up call not only to Aaron but to the rest of the Israelites. If God was willing to kill his own priests for as minor a problem as burning the wrong incense, what would he do for more serious problems? Unfortunately, the rest of Biblical history seems to be a repeated loop of the Israelites forgetting this early lesson, sinning, and being punished. La plus ca change...