By fire, God killed Aaron's sons, Nadav and Avihu, for the sin of bringing an offering to the Tabernacle when they were not told to do so. It was right after their death, of all times, that God chose to tell Aaron about the ritual on Yom Kippur for purifying the Tabernacle of sin.  

If the timing of this instruction was painful for Aaron— like God rubbing salt into the wounds of his grief—the symbolism of the scapegoat ritual may have been even more agonizing. God commanded Aaron, every year, to watch as the pair were symbolically cast into fire again.

"Bring two goats to the Tent of Meeting," God tells Aaron (Lev. 16:7-8). "Place lots on them, one marked 'for the Lord' and one marked 'for Azazel.'" The one marked 'for the Lord' was offered as a sacrifice upon the altar to atone for sin. The one marked 'for Azazel' was sent off into the wilderness, never to be seen again, to symbolically carry away the people's sins.

Aaron doubly experienced the worst possible grief, the death of a child, and was doomed to recapitulate it every Yom Kippur. Those two goats, marked for death because of sin, might as well have been the ghosts of Nadav and Avihu for Aaron. 

Perhaps Aaron wondered which of his boys was meant "for the Lord" and which "Azazel." Could it have mattered? Both of them were dead.

What (or who) is Azazel? No explanation is offered in the Torah. Azazel has been explained variously as a vestigial reference to a demon, a place of impurity, a personification of wickedness, or simply as a word that means "sent away."  In the end, it does not really matter what the word meant originally. We understand it as the place where our sins go once we wriggle loose of them. 

The rabbis of the Mishnah elaborated on the scapegoat ritual and say that the High Priest would tie a piece of red wool onto the head of the goat sent to Azazel and that he would "turn in the direction to which it was sent" (M. Yoma 4:1-3). And so, I imagine, Aaron, for many years after the deaths of his two oldest sons, staring off into the wilderness on Yom Kippur, searching for the place where the goat was sent. I hear him muttering: Are you there, Nadav?  Avihu, my boy, are you there? Is that bit of red I see on the horizon the ribbon I tied on your hand the day you were born? How can it be that I have wriggled loose of you?

Regret and grief are not feelings we associate with joy, but they are a necessary part of living in a world of imperfection and impermanence. In order for us, who have lived through sorrow, to come back to joy, we have to be willing to look in the direction of our loss. We have to be able to acknowledge the hard pit of grief in our gut that will never go away. It is only after we have turned in the direction of Azazel, that we can turn back to living life with appreciation of what we have, and what we had.