Nadav and Avihu, two sons of Aaron, were consumed by fire on the day of the inauguration of the Mishkan. Why did this happen? There are various reasons given. Rabbi Soloveitchik gives a very unique answer. He does not quote all the Midrashic explanations but refers directly to the reason the Torah mentions.
The Torah states they each took a fire pan and put fire and incense on them and brought before Hashem an alien fire: אשר לא צוה אתם, “…that He had not commanded them.” (Lev. 10,1) They were overcome with a high spiritual emotion and brought the identical incense their father Aharon had brought. What was there transgression?
Rabbi Soloveitchik suggests that what they did wrong was that in their ecstasy and religious fervor they brought incense that was not commanded by Hashem. Religious observances in Judaism are not the initiative of individuals. Our faith is ordained by Hashem. It is not created by the imagination of individuals.
The Torah commands us to perform certain Mitzvot. They constitute our religious observance. Anything not decreed in the Torah and that is not based on Torah values is not a legitimate expression of Jewish faith. This was the error of Nadav and Avihu. Their intentions were commendable but they created their own rules and did not act within accord with the commands of Hashem.