Albert Barnes Commentary James 2:11

"For he that said, Do not commit adultery, said also, Do not kill. Now if thou dost not commit adultery, but killest, thou art become a transgressor of the law." — James 2:11 (ASV)

For he that said, Do not commit adultery, said also, Do not kill. That is, these are parts of the same law of God, and one is as obligatory as the other. If, therefore, you violate either of these precepts, you transgress the law of God as such, and must be held to be guilty of violating it as a whole. The penalty of the law will be incurred, whatever precept you violate.

"For he that said," or, "that law which said," Do not commit adultery, said also, Do not kill (Exodus 20:13–14).