The Bais Avrohom of Slonim zy”a (Kovetz Sichos Kodesh) explains why Moshe Rabenu was commanded to bring a Korban Chatas first by quoting the famous words of the Apta Rov zy”a that his own soul had already been in this world four times previously. His first incarnation was as a sheep in Yaakov’s flock, as all of these sheep had lofty Jewish souls.
His second time in this world was as Shlomo Hamelech. His third time was as Rav Yishmoel Kohen Gadol, and his fourth time was as a member of a group of kohanim who served in the Bais Hamikdosh.
He related a story that occurred when he was a member of that group of kohanim: There was a distinguished resident of Yerushalaim named R’ Groinem who accidentally committed a sin and needed to bring a Korban Chatas. He went to the marketplace to purchase an animal for the korban. Since all the cattle sellers knew that male animals are used for Korban Olah and females are used for Chatas, when R’ Groinem asked for a female, they realized that it was meant to be a Korban Chatas and asked, “How could it be that a man like you, R’ Groinem, has to bring a Chatas?”
He then had to lead the animal through the streets of Yerushalaim to the Bais Hamikdosh, and everyone he met asked the same question. This was all extremely embarrassing for him. When he finally arrived at the entrance to the Bais Hamikdosh, the animal ran away. He had to chase it through the streets until he caught it, which caused him even more embarrassment.
After all of this, no kohen wanted to be the one to offer his korban. The rule is that the kohanim eat the korban and the owner receives his atonement, but no kohen wanted to eat the meat of a korban that was brought for a sin. It took much convincing until he found one kohen who agreed to offer the korban.
The sign they had to know if the korban was accepted by Hashem was if the smoke formed the shape of a lion. If it formed the shape of a dog, it was a sign that it had not been accepted. When R’ Groinem’s korban was placed on the fire, the smoke formed the shape of a dog. He began to cry bitterly until the image changed and began to look like a lion.
We see from this story that the atonement of a Chatas is not simply from bringing the korban and shedding the blood of an animal. Rather, the atonement comes from the heart of the one offering it breaking within him. When the animal’s blood is spilled, it is symbolic of the owner receiving atonement as if his own blood were pouring out of him. Accordingly, Moshe commanded Aharon to first bring a Korban Chatas to atone for the sin of the eigel. Since he was connected to this sin, his heart was broken over it, and he could offer this korban to atone for all of Klal Yisroel.
