וחְקִיַו יֵנְפִל וביִרְקַיַו תֶׁרֹטְק ָהיֶׁלָע ומיִ שָיַו שֵא ןֵהָב ונְתִיַו ֹותָתְחַמ שיִא א והיִבֲאַו בָדָנ ןֹרֲהַא יֵנְבה' אֵש זָרָ ה אֲש ֶׁר לֹא םָתֹא הָוִצ...
Nadav and Avihu, the sons of Aaron, each took his censer, and put fire in it, and laid incense on it, and offered strange fire before Hashem, which he had not commanded them.
The Ohr Hachaim explains that Nadav and Avihu had two mistaken thoughts that caused them to offer up the strange Ketores and die. This is the reason they are given two descriptions in this possuk, first being entitled ‘the sons of Aaron’, and only then referred to by name.
First, they were Cohanim, whose job it was to serve Hashem. This is why they were called the children of Aaron. They felt that they could add to their obligations, because they were now classified as servants of Hashem, more than the rest of Klal Yisroel.
Another mistake they made was that they were the greatest of the children of their father. Nadav and Avihu were greater than Elazar and Isamar, equal to Moshe and Aaron. The Zohar writes that they were on a higher level because of their actions, not merely because they were born first. They felt that it would be appropriate for them to offer a Korban over and above that which their brothers offered.
They used these two mistakes to decide to bring their own private Ketores, and this is why they were punished.
They then entered the Mishkan together, not separately. This was to increase their merits and spiritual level. This is why the Torah refers to them in the plural, at first.
They did not wish to take fire from the Mizbeach, which is why the Torah says simply שֵׁא ןֵהָב וּנְּתִּיַו – and they put fire on it. They were aware that there was a difference between the fire that was used for Ketores that was obligatory as opposed to the fire that was used for Ketores that was voluntary. Although the daily obligatory Ketores was burned in a fire that came from the Mizbeach, they did not use that fire. They were just unaware that voluntary Ketores was completely invalid.
The possuk writes וַיָּשִימוּ עָלֶיהָ קְטֹרֶת - and they placed Ketores on it. They placed the Ketores on the fire, not the shovels. This tells us that the main problem with their actions was the fact that they used strange fire, their own fire. If they would have taken fire from the Mizbeach, Hashem would not have punished them.
Another explanation offered by the Ohr Hachaim is that the problem with the fire was the fact that they were not commanded to do so. The fire is a strange fire, because it was not commanded.
