4:8 talked with Abel. Abel was a prophet and no doubt urged Cain to repent and believe God’s Word, but this only angered Cain further. The Serpent was quickly striking at the Seed of the woman, corrupting her first son and slaying her second, trying to prevent the fulfillment of the Protevangelic promise.
4:9 I know not. Cain thus added blatant lying to his sins of self-righteous pride and murder. However, in one sense, he was speaking the truth. He knew where Abel’s blood was spilled but not where Abel himself was. Abel was now the first human inhabitant of Sheol (or Hades), that place in the heart of the earth where departed spirits would reside while awaiting the coming of the Savior (Luke 16:22-26; Ephesians 4:8-10; I Peter 3:18-20).
4:10 thy brother’s blood. This first mention of “blood” in Scripture prefigures the innocent blood of Christ, which “speaketh better things than that of Abel” (Hebrews 12:24). The voice of Abel’s blood cried for vengeance (compare Revelation 5:9, 10), but the blood of Christ speaks of cleansing and forgiveness (I John 1:7; Ephesians 1:7).
4:11 cursed from the earth. The earth had been cursed because of Adam’s sin; now the earth itself had been defiled by Cain’s sin. God’s curse was on the earth; Cain’s curse was from the earth. His boastful pride in the fruits he had been able to grow from the cursed earth had been the occasion of his sin, but now he would no longer be able to till the ground even for his own food. Those who trust in their own good works eventually find it impossible to produce them any more.