M. Weisman, “The Midrash Says” Chapter “Shemini“: The moment that Nadav and Avihu decided that they should bring their own fire to the altar it says: ‘And Nadav and Avihu, the sons of Aaron, took each of them his censor, and put fire therein, and laid incense thereon, and offered strange fire before the Lord.’
The sages say that they were guilty of several breakings of the law. As was already said, they did not consult with Moses and Aaron and not even with each other first, and each one acted independently.
The right and the left lines (Nadav and Avihu) have to be inclined to try to connect to the middle line, but they didn’t do so.
Nadav and Avihu were attracted by their strong passion to actually feel the presence of the Almighty, and entered the Holy of Holiness in order to bring the censor there, etc. This means that they wanted to reveal the Creator without a Masach (screen) and the Returning Light.
Only the great priest can enter the Holy of Holiness, since he has a Masach. Therefore he is called the great priest, which means the holy priest who works in bestowal above others.
Nadav and Avihu didn’t have the attribute of bestowal (the Returning Light) and each one worked on his own: the right line separately from the left line. However, the Returning Light appears only on the middle line in the right connection between the two lines.
Question: Does that mean that they didn’t have a Masach on the pleasures that they wanted to receive?
Answer: The revelation of the Creator is an immense pleasure! This is actually the goal of creation. So the goal of the wisdom of Kabbalah is to reveal the Creator to everyone, by filling everyone with great pleasure.
Besides, the “strange fire” that Nadav and Avihu brought indirectly indicates other sins.
They drew the Light of Hochma that couldn’t be dressed in the Light of Hassadim (Light of Mercy). Without this dressing, the Light of Hochma doesn’t warm, and doesn’t save but burns. Nadav and Avihu lacked the Light of Hassadim in order to receive such great Light. They didn’t even try to receive it since they worked separately.
From KabTV’s “Secrets of the Eternal Book” 1/15/14