Q. Aaron’s sons Nadab and Abihu offered unauthorized fire before the Lord, and were consumed by fire before God. Their brothers Eleazar and Ithamar offered a sin offering but did not eat it. Why was Moses angry with them, then changed his mind?
A. The incident referred to is in Lev 10:
• Lev 10:16-20 But Moses searched carefully for the goat of the sin offering, and behold, it had been burned up! So he was angry with Aaron’s surviving sons Eleazar and Ithamar, saying, “Why did you not eat the sin offering at the holy place? For it is most holy, and He gave it to you to bear away the guilt of the congregation, to make atonement for them before the LORD. Behold, since its blood had not been brought inside, into the sanctuary, you should certainly have eaten it in the sanctuary, just as I commanded.” But Aaron spoke to Moses, “Behold, this very day they presented their sin offering and their burnt offering before the LORD. When things like these happened to me, if I had eaten a sin offering today, would it have been good in the sight of the LORD?” When Moses heard that, it seemed good in his sight.
The rules governing the sin offering are given in Lev 6:24-30:
• Lev 6:24-26, 29-30 Then the LORD spoke to Moses, saying, “Speak to Aaron and to his sons, saying, ‘This is the law of the sin offering: in the place where the burnt offering is slain the sin offering shall be slain before the LORD; it is most holy. The priest who offers it for sin shall eat it. It shall be eaten in a holy place, in the court of the tent of meeting. … Every male among the priests may eat of it; it is most holy. But no sin offering of which any of the blood is brought into the tent of meeting to make atonement in the holy place shall be eaten; it shall be burned with fire.
There were two types of sin offering:
1. Those whose blood was brought into the sanctuary – the meat should be burned;
2. Those whose blood was not brought into the tabernacle – the meat should be eaten.
In this case the blood had not been brought into the sanctuary, so Eleazar and Ithamar should have eaten the sin offering. Moses was angry because they were clearly in violation of Lev 6:25-26. However, Aaron defended them by saying that they did everything else, but could not eat today i.e. just this time, because they were mourning for their sons/brothers by fasting. They were not deviating from the rule by being lax. Moses accepted his explanation and did not pursue further. Hope this helps.