Leviticus 10:1 - The Popular Commentary by Paul E. Kretzmann

And Nadab and Abihu, the sons of Aaron, took either of them his censer, and put fire therein, and put incense thereon, and offered strange fire before the Lord, which he commanded them not. Nadab and Abihu were probably the oldest sons of Aaron, and were distinguished by Moses in being selected to accompany him to the feast before the Lord, Exodus 24:1-9. The chances are that they were unduly exalted over their initiation into the priesthood and believed themselves competent to select their own methods of worshiping the Lord. They took two of the small vessels pertaining to the golden altar, put some fire in them which was not taken from the altar of burnt offering, and proceeded to offer incense to the Lord in these censers of their own selecting, whereas incense was to be offered only on the golden altar by the officiating priest twice daily, Exodus 30:7-8.