Charles Ellicott Commentary Isaiah 66:3

"He that killeth an ox is as he that slayeth a man; he that sacrificeth a lamb, as he that breaketh a dog`s neck; he that offereth an oblation, [as he that offereth] swine`s blood; he that burneth frankincense, as he that blesseth an idol. Yea, they have chosen their own ways, and their soul delighteth in their abominations:" — Isaiah 66:3 (ASV)

He that killeth an ox ... —The truth of the previous verse is emphasized by iteration, each clause presenting a distinct illustration of it. Isaiah 65:3–11 had pointed to tendencies, not yet extinct, which led to open apostasy. Now the prophet declares that there may be as real an apostasy beneath an orthodox creed and an irreproachable ritual. Each act of the hypocrite’s worship is as an idolatrous abomination.