Try this from the Haydock Bible Commentary Genesis 9:4 and 9:6
Ver. 4. With blood. This was a matter of indifference in itself, like the forbidden fruit. But God gave the prohibition, to keep people at a greater distance from imbruing their hands in the blood of others, which nevertheless we know some have drunk! He would also assert his dominion over all things; the blood or life of animals being reserved to be offered in sacrifice to him, instead of the life of man, Leviticus xvii. 11. Blood of brutes is gross and unwholesome. (Menochius) — The apostles required this law to be observed by the first Christians, that the Jews might not be disgusted: but, after a competent time had been allowed them, the Church thought proper to alter this discipline. (St. Augustine, contra Faust. xxxii. 13.)
Ver. 6. Shed. God had not subjected Cain to this law of retaliation, as he was the first murderer, and the earth was unpeopled. (Haydock) — Here he declares, that it is just to inflict such a punishment on the offender. (Menochius) — Judges are hence authorized to punish murderers with death. (Calmet) — The general law, thou shalt not kill, admits of exceptions, and forbids killing by private authority, or out of revenge. (Haydock) — The blood of your lives, may signify the blood on which your life depends; or, according to the Rabbin, it is a prohibition of suicide, which one would think is so contrary to the first law of nature, self-preservation, as to require no prohibition; and yet, to the scandal of philosophers, some have written in its defence! (Haydock)
