Matthew Henry Commentary Leviticus 10:1-2

"And Nadab and Abihu, the sons of Aaron, took each of them his censer, and put fire therein, and laid incense thereon, and offered strange fire before Jehovah, which he had not commanded them. And there came forth fire from before Jehovah, and devoured them, and they died before Jehovah." — Leviticus 10:1-2 (ASV)

Next to Moses and Aaron, none were more likely to be honourable in Israel than Nadab and Abihu. There is reason to think that they were puffed up with pride, and that they were heated with wine. While the people were prostrate before the Lord, adoring his presence and glory, they rushed into the tabernacle to burn incense, though not at the appointed time; both together, instead of one alone, and with fire not taken from the altar.

If it had been done through ignorance, they had been allowed to bring a sin-offering. But the soul that doeth presumptuously, and in contempt of God's majesty and justice, that soul shall be cut off. The wages of sin is death. They died in the very act of their sin.

The sin and punishment of these priests showed the imperfection of that priesthood from the very beginning, and that it could not shelter any from the fire of God's wrath, except as it was typical of Christ's priesthood.