A Wondrous Revelation from the Holy Ohr HaChaim on the Passing of Nadav and Avihu: Even as They Sensed Their Impending Death, They Yearned for the Divine

Alternatively, we can explain the verse, "After the death..." (Leviticus 16:1), as Hashem speaking to Moshe about the manner of their death. Their passing occurred "when they drew near before Hashem." This means they drew so close to the Supernal Light out of a holy, burning love, and through this, they died. This is the secret of the "Divine Kiss" (Mitas Neshika) through which tzaddikim pass away.
In this respect, their death was identical to the passing of all tzaddikim. The only difference is that for other tzaddikim, the Divine Kiss draws near to them, whereas Nadav and Avihu actively drew near to it. This is the meaning of the words, "when they drew near before Hashem."
The verse adds the letter Vav ("and they died") to hint at the wondrous love of these tzaddikim. Even though they sensed that they were dying, they did not hold themselves back from drawing closer to the dveikus (cleaving to Hashem)—the pleasantness, the sweetness, the friendship, the endearment, the yearning, and the absolute delight—until their souls literally expired from within them. Understand this well.
The true quality of this spiritual level cannot be recognized; it is entirely beyond human comprehension. It cannot be conveyed through human speech or writing, nor can it be grasped through physical, intellectual estimation. If someone manages to attain even a fraction of this perception, it will separate him from anything that prevents him from receiving it. You will recognize it by its signs, for it is the exact opposite of its adversaries and enemies (the physical desires). It will leave a profound mark on him, stripping away the power of that which holds him back and anchors him to this world. Sometimes, the soul will become utterly disgusted with the physical body, engaging in a massive struggle against it. This is the secret of the "madness" (ecstatic frenzy) of the prophets (I Samuel 19:24). When this level intensifies within a person's innermost being, the soul will loathe the flesh, depart from it, and return to her Father's house.
I will now reveal a secret to the one who contemplates the inner essence of spiritual comprehension: The true understanding of the intellect illuminates all other understandings. When one truly comprehends his own perception, he will realize that the Ultimate Known (Hashem) is completely stripped of human intellect. When he perceives this through his own awakening—realizing that it is not his own self doing the comprehending—he will understand that this perception is derived from a Higher Perception, entirely independent of human logic. This grants the enlightened ones a unified perception, the secret of the "soul of the soul." Then, even their physical remnants will become like the crowns and thrones of kings. They will realize that there is a "Life within life," about which Moshe Rabbeinu said, "Choose the life" (Devarim 30:19)—with the definitive letter Hei (the specific, ultimate life). This negates the ordinary, physical life that is universally experienced. And they will bless the Living God who has reserved this treasured spiritual quality for His treasured nation.
It is brought down in the sefer Arvei Nachal that the holy Ohr HaChaim wrote these words with Ruach HaKodesh (Divine Inspiration).