M.Weisman, Midrash Sefer, “Shemini:” Nadav and Avihu lost their lives since they regarded the laws of the Holy of Holies light headedly. The nation was shocked by the serious punishment. It was an indication that the Creator will not allow any changes or even the slightest innovations in the ceremony that was dedicated to Him.
Everything is done according to the laws of nature and therefore whoever is wrong, even in the good sense, by wanting to serve the Creator and trying to intensify certain actions especially on the high levels, he commits a sin and, in fact, immediately falls from that level.
What is more, his fall is identical to his height. It is spiritual death and absolute detachment from the previous level. This is a fall to a very low level where the shattering begins followed by the correction beginning again by connection and unity.
This is our spiritual desire, Nadav and Avihu, the desire to breakthrough and to leap up all the levels as quickly as possible.
The sons of Aaron, Nadav and Avihu, had to ascend higher than Aaron, and they did so, but it was before the correct time to do so.
From KabTV’s “Secrets of the Eternal Book” 1/22/1