Charles Ellicott Commentary Leviticus 8:15

"And he slew it; and Moses took the blood, and put it upon the horns of the altar round about with his finger, and purified the altar, and poured out the blood at the base of the altar, and sanctified it, to make atonement for it." — Leviticus 8:15 (ASV)

And he slew it. —Better, and he killed it, as it is rendered in the Authorised Version in Leviticus 8:19. In ordinary cases, the offerer himself slaughtered the victim , but in this case, Moses performed this act in accordance with the command in Exodus 29:11.

And Moses took the blood. — That is, having caught the blood in the bowl, he threw it on the four corners of the altar, as described in Leviticus 1:5 — not, however, on the horns of the altar of incense, or in the tabernacle, as in the case of the sin offering for the high priest and for the nation (Leviticus 4:16–18).

And purified the altar ... and sanctified it. — Like the priest, the altar was consecrated to the service of God by the anointing oil , and therefore, like the priest, the altar is also purified by the expiatory sacrifice from its defilements.