Nadab and Abihu
In chapter 9 Moses instructs Aaron to proceed and offer the first offerings as God's high priest. In verse 15, the offering for the people is a goat. While the animal specified as a sin offering for the congregation in Leviticus 4:14 Leviticus 4:14When the sin, which they have sinned against it, is known, then the congregation shall offer a young bullock for the sin, and bring him before the tabernacle of the congregation.
American King James Version×was a bull, a goat was used for this purpose on some occasions (Leviticus 16:9 Leviticus 16:9And Aaron shall bring the goat on which the LORD's lot fell, and offer him for a sin offering.
American King James Version×; Leviticus 5:6 Leviticus 5:6And he shall bring his trespass offering to the LORD for his sin which he has sinned, a female from the flock, a lamb or a kid of the goats, for a sin offering; and the priest shall make an atonement for him concerning his sin.
American King James Version×; Numbers 28:15 Numbers 28:15And one kid of the goats for a sin offering to the LORD shall be offered, beside the continual burnt offering, and his drink offering.
American King James Version×; Numbers 29:16 Numbers 29:16And one kid of the goats for a sin offering; beside the continual burnt offering, his meat offering, and his drink offering.
American King James Version×; Numbers 15:22-26 Numbers 15:22-26  And if you have erred, and not observed all these commandments, which the LORD has spoken to Moses,  Even all that the LORD has commanded you by the hand of Moses, from the day that the LORD commanded Moses, and henceforward among your generations;  Then it shall be, if ought be committed by ignorance without the knowledge of the congregation, that all the congregation shall offer one young bullock for a burnt offering, for a sweet smell to the LORD, with his meat offering, and his drink offering, according to the manner, and one kid of the goats for a sin offering.  And the priest shall make an atonement for all the congregation of the children of Israel, and it shall be forgiven them; for it is ignorance: and they shall bring their offering, a sacrifice made by fire to the LORD, and their sin offering before the LORD, for their ignorance:  And it shall be forgiven all the congregation of the children of Israel, and the stranger that sojournes among them; seeing all the people were in ignorance.
American King James Version×; 2 Chronicles 29:20-24 2 Chronicles 29:20-24  Then Hezekiah the king rose early, and gathered the rulers of the city, and went up to the house of the LORD.  And they brought seven bullocks, and seven rams, and seven lambs, and seven he goats, for a sin offering for the kingdom, and for the sanctuary, and for Judah. And he commanded the priests the sons of Aaron to offer them on the altar of the LORD.  So they killed the bullocks, and the priests received the blood, and sprinkled it on the altar: likewise, when they had killed the rams, they sprinkled the blood on the altar: they killed also the lambs, and they sprinkled the blood on the altar.  And they brought forth the he goats for the sin offering before the king and the congregation; and they laid their hands on them:  And the priests killed them, and they made reconciliation with their blood on the altar, to make an atonement for all Israel: for the king commanded that the burnt offering and the sin offering should be made for all Israel.
American King James Version×; Ezra 6:17 Ezra 6:17And offered at the dedication of this house of God an hundred bullocks, two hundred rams, four hundred lambs; and for a sin offering for all Israel, twelve he goats, according to the number of the tribes of Israel.
American King James Version×; Ezra 8:35 Ezra 8:35Also the children of those that had been carried away, which were come out of the captivity, offered burnt offerings to the God of Israel, twelve bullocks for all Israel, ninety and six rams, seventy and seven lambs, twelve he goats for a sin offering: all this was a burnt offering to the LORD.
American King James Version×).
At this inauguration of sacrifices, Aaron pronounces a blessing on Israel (verse 22). The specific wording of the priestly blessing that God commanded to be bestowed upon Israel is given in Numbers 6:23-26 Numbers 6:23-26  Speak to Aaron and to his sons, saying, On this wise you shall bless the children of Israel, saying to them,
 The LORD bless you, and keep you:
 The LORD make his face shine on you, and be gracious to you:
 The LORD lift up his countenance on you, and give you peace.
American King James Version×. This may be the blessing to which Leviticus 9:22 Leviticus 9:22And Aaron lifted up his hand toward the people, and blessed them, and came down from offering of the sin offering, and the burnt offering, and peace offerings.
American King James Version×refers.
In verses 23-24 we see a spectacular event. "The sacrifices were consumed, not by fire ignited by Aaron, but by fire from before the Lord. This is the first of only five times that the Old Testament records fire from God as a sign that a sacrifice was accepted (Judges 6:21 Judges 6:21Then the angel of the LORD put forth the end of the staff that was in his hand, and touched the flesh and the unleavened cakes; and there rose up fire out of the rock, and consumed the flesh and the unleavened cakes. Then the angel of the LORD departed out of his sight.
American King James Version×; 1 Kings 18:38; 1 Chronicles 21:26; 2 Chronicles 7:1). Since the fire on this altar was never to go out [see Leviticus 6:9 Leviticus 6:9Command Aaron and his sons, saying, This is the law of the burnt offering: It is the burnt offering, because of the burning on the altar all night to the morning, and the fire of the altar shall be burning in it.
American King James Version×, 12-13], all Israel's sacrifices from this time forward would be consumed by fire that originated from God" (Nelson Study Bible, note on 9:24). However, while certainly plausible, it is not absolutely clear that this was the case.
After Aaron's sons are later killed for bringing profane fire before the Lord, Moses explains to Aaron why God has done this and then instructs Aaron's cousins to remove the dead men from the sanctuary. God then commands Aaron and his sons to not drink alcohol before going into the tabernacle of meeting. But the account had only spoken of Nadab and Abihu bringing profane fire and incense before God—so why is this particular instruction regarding intoxicating drink given to Aaron in the midst of what had just happened? Although it is possible that God was simply relating another way that one could show disregard for him during these rituals, the text here may be indicating that the inappropriate use of alcohol had played a role in the two brothers' poor judgment and behavior.
The punishment God inflicted on the two was very severe. We know there are certainly many times where people have "worshiped" God in a way that He does not recognize or appreciate, yet for which He does not strike them down immediately. However, at the time of this account, God was playing a very visible role in the nation of Israel and was actually teaching the people the magnitude of reverence they needed to have for Him: "By those who come near Me I must be regarded as holy; and before all the people I must be glorified" (Leviticus 10:3 Leviticus 10:3Then Moses said to Aaron, This is it that the LORD spoke, saying, I will be sanctified in them that come near me, and before all the people I will be glorified. And Aaron held his peace.
American King James Version×)—it was critical for them to understand.
What Aaron's sons did was not in ignorance, for God had already given clear instructions through Moses on how He was to be regarded. In this situation, Nadab and Abihu's disregard and carelessness could not go uncorrected—it was not only offensive to God, but would have fostered a careless attitude about God's instructions among the people. When God says to regard Him as holy, He means it. The instructive nature of this event was so important that Aaron and his remaining sons were not allowed to show any outward sign of grievance—they were required to maintain their composure and to continue their priestly duties to illustrate the justice and righteousness of God's wrath.
The NIV Study Bible notes regarding the death of Nadab and Abihu: "They are regularly remembered as having died before the Lord and as having had no sons. Their death was tragic and at first seems harsh, but no more so than that of Ananias and Sapphira (Acts 5:1-11 Acts 5:1-11  But a certain man named Ananias, with Sapphira his wife, sold a possession,
 And kept back part of the price, his wife also being privy to it, and brought a certain part, and laid it at the apostles' feet.
 But Peter said, Ananias, why has Satan filled your heart to lie to the Holy Ghost, and to keep back part of the price of the land?
 Whiles it remained, was it not your own? and after it was sold, was it not in your own power? why have you conceived this thing in your heart? you have not lied to men, but to God.
 And Ananias hearing these words fell down, and gave up the ghost: and great fear came on all them that heard these things.
 And the young men arose, wound him up, and carried him out, and buried him.
 And it was about the space of three hours after, when his wife, not knowing what was done, came in.
 And Peter answered to her, Tell me whether you sold the land for so much? And she said, Yes, for so much.
 Then Peter said to her, How is it that you have agreed together to tempt the Spirit of the Lord? behold, the feet of them which have buried your husband are at the door, and shall carry you out.
 Then fell she down straightway at his feet, and yielded up the ghost: and the young men came in, and found her dead, and, carrying her forth, buried her by her husband.
 And great fear came on all the church, and on as many as heard these things.
American King James Version×). In both cases a new era was being inaugurated.... The new community had to be made aware that it existed for God, not vice versa."
Moses pointing out that the goat of the sin offering (Leviticus 10:16 Leviticus 10:16And Moses diligently sought the goat of the sin offering, and, behold, it was burnt: and he was angry with Eleazar and Ithamar, the sons of Aaron which were left alive, saying,
American King James Version×) was not to be burned but eaten by the priests shows that this particular sin offering was not for the whole congregation or priesthood (see Leviticus 4). It is thus a later offering than the one referred to in Leviticus 9:15 Leviticus 9:15And he brought the people's offering, and took the goat, which was the sin offering for the people, and slew it, and offered it for sin, as the first.
American King James Version×. Following the death of his nephews, Moses was rather concerned about making sure everything was being done correctly. In verse 18, he isn't rebuking Aaron's sons for failing to bring the blood into the holy place, but rather pointing out that, because the blood was not brought in, the offering was to be eaten, not burned (see Leviticus 6:29-30 Leviticus 6:29-30  All the males among the priests shall eat thereof: it is most holy.  And no sin offering, whereof any of the blood is brought into the tabernacle of the congregation to reconcile with in the holy place, shall be eaten: it shall be burnt in the fire.
American King James Version×).
Aaron explains that he himself did not eat of the offering because he was afraid God would not accept it. Eating of the sin offering was an act of worship symbolizing satisfaction with God's justice, and Aaron understood the need to be in a proper and reverential frame of mind. Yet he and his sons were sorely grieved and distracted by what had happened—perhaps even unnerved and unhappy with God's judgment for the moment.
"Aaron did not eat of the sacrificial meat because he was afraid of what more God might do. He was not being rebellious, as his dead sons had been in burning the incense. Aaron was arguing that in circumstances such as the one he faced that day, God would prefer the priest to err on the side of caution rather than presumption.... Rebellion arises from a heart that is not right toward God. Moses recognized that Aaron's failure was not rebellion, that his argument had merit, and that Aaron could be forgiven" (Nelson Study Bible, notes on verses 19-20).