Moses
Genesis 4:9ESV·traditional attribution

Then the LORD said to Cain, “Where is Abel your brother?” He said, “I do not know; am I my brother’s keeper?”

John Calvin Reformed @genevareformer

9. Where is Abel ? They who suppose that the father made this inquiry of Cain respecting his son Abel, enervate the whole force of the instruction which Moses here intended to deliver; namely, that God, both by secret inspiration, and by some extraordinary method, cited the parricide “Parricidam citaverit.” The word parricide is contrary to its original import, applied to the murderer of any near relative. — Ed.

Matthew Henry Presbyterian @wholebiblehenry

We have here a full account of the trial and condemnation of the first murderer. Civil courts of judicature not being yet erected for this purpose, as they were afterwards (Gen 9:6), God himself sits Judge; for he is the God to whom vengeance belongs, and who will be sure to make inquisition for blood, especially the blood of saints. Observe, I.

Commenting on Genesis 4:9-12

John Gill Reformed Baptist @doctorgill

And he said,.... Not Cain, the last speaker, but the Lord God: what hast thou done? what an heinous crime hast thou committed! how aggravated is it! I know what thou hast done; thou hast slain thy brother, thine own, thine only brother, a holy, righteous, and good man, who never gave thee any offence, or any just occasion of shedding his innocent blood: this...