So Aaron drew near to the altar and killed the calf of the sin offering, which was for himself.
These being the first offerings that ever were offered by the levitical priesthood, according to the newly-enacted law of sacrifices, the manner of offering them is particularly related, that it might appear how exactly they agreed with the institution. 1.
Commenting on Leviticus 9:8-22
Aaron therefore went unto the altar,.... Of burnt offering, freely and cheerfully, at the direction and introduction of Moses, who acted in this affair in the name of the Lord: and slew the calf of the sin offering, which was for himself; which was to be offered first, as it was proper it should, that, atonement being made for his sins, his after burnt offering...
Aaron . . . went unto the altar, and slew the calf of the sin offering--Whether it had been enjoined the first time, or was unavoidable from the divisions of the priestly labor not being as yet completely arranged, Aaron, assisted by his sons, appears to have slain the victims with his own hands, as well as gone through all the prescribed ritual at the altar.