On Submission to Justice
Then Nadab and Abihu, the sons of Aaron, each took his censer and put fire in it, put incense on it, and offered profane fire before the LORD, which he commanded them not. And there went out fire from the LORD, and devoured them, and they died before the LORD. And Moses said to Aaron, “This is what the Lord spoke, saying: ‘By those who come near Me I must be regarded as holy; and before all the people I must be glorified.’” So Aaron held his peace. Leviticus 10:1-3
It does not appear that Nadab or Abihu had any orders to burn incense at all at this time. They completed their consecration the day before and it was part of their work as priests to serve at the altar of incense, but it seems that the whole service of this solemn day of inauguration was to be performed by Aaron himself. For he slew the sacrifices and his sons were only to attend him (Leviticus 10:9,12,18), therefore Moses and Aaron only went into the tabernacle (Leviticus 10:23). But Nadab and Abihu were so proud of the honor they were newly advanced to and so ambitious of doing the highest and most honorable part of their work immediately, that though the service of this day was extraordinary and done by particular direction from Moses, yet without receiving orders or so much as asking leave from him, they took their censers entered into the tabernacle and burned incense. The priests were to burn incense only when it was their lot (Luke 1:9). At this time, it was not their turn.
Moses was composed, not being displeased, as David was in a similar case, (2 Samuel 6:8) But though it touched him in a very tender part and was a dreadful damp to one of the greatest joys he ever knew, yet he kept possession of his own soul and took care to keep good order and a due decorum in the sanctuary.
He also endeavoured to pacify Aaron and to keep him in a good attitude under this sad news. Moses was a brother that was born for adversity. He has taught us by his example with counsel and comfort to support the weak and strengthen the feeble-minded.
Moses suggested to his poor brother upon this occasion that, “This is it that the Lord spoke.” Note that the most calming considerations under affliction are those that come from the Word of God.
Though Aaron's heart must have been filled with anguish and dismay, yet with silent submission he understood with reverence the justice of the stroke. When God corrects us for sin, it is our duty to accept the punishment and say, “It is the Lord, let Him do what seems good.”
We should practice holiness, trusting in God's justice and holding our peace because every situation is ordained by God.
Righteousness will go before Him, and shall make His footsteps our pathway. Psalm 85:13
The above information was abridged from Matthew Henry's Commentary On The Whole Bible