Genesis 4:8 Cain spoke to Abel his brother. And when they were in the field, Cain rose up against his brother Abel and killed him. Then the LORD said to Cain, “Where is Abel your brother?” He said, “I do not know; am I my brother’s keeper?” And the LORD said, “What have you done? The voice of your brother’s blood is crying to me from the ground. And now you are cursed from the ground, which has opened its mouth to receive your brother’s blood from your hand. When you work the ground, it shall no longer yield to you its strength. You shall be a fugitive and a wanderer on the earth.” Cain said to the LORD, “My punishment is greater than I can bear. Behold, you have driven me today away from the ground, and from your face I shall be hidden. I shall be a fugitive and a wanderer on the earth, and whoever finds me will kill me.” Then the LORD said to him, “Not so! If anyone kills Cain, vengeance shall be taken on him sevenfold.” And the LORD put a mark on Cain, lest any who found him should attack him. Then Cain went away from the presence of the LORD and settled in the land of Nod, east of Eden.
1 John 3:12-15 We should not be like Cain, who was of the evil one and murdered his brother. And why did he murder him? Because his own deeds were evil and his brother’s righteous. Do not be surprised, brothers, that the world hates you.
Jude 1:11 Woe to them! For they walked in the way of Cain and abandoned themselves for the sake of gain
Mat 23:33-35 You serpents, you brood of vipers, how are you to escape being sentenced to hell?Therefore I send you prophets and wise men and scribes, some of whom you will kill and crucify, and some you will flog in your synagogues and persecute from town to town, so that on you may come all the righteous blood shed on earth, from the blood of righteous Abel to the blood of Zechariah the son of Barachiah, whom you murdered between the sanctuary and the altar.
Malice in the heart ends in murder by the hands. Cain slew Abel, his own brother, his own mother’s son, whom he ought to have loved; his younger brother, whom he ought to have protected; a good brother, who had never done him any wrong. What fatal effects were these of our first parents’ sin, and how must their hearts have been filled with anguish! Observe the pride, unbelief, and impenitence of Cain. He denies the crime, as if he could conceal it from God. He tries to cover a deliberate murder with a deliberate lie. Murder is a crying sin. Blood calls for blood, the blood of the murdered for the blood of the murderer. Who knows the extent and weight of a Divine curse, how far it reaches, how deep it pierces? Only in Christ are believers saved from it, and inherit the blessing. Cain was cursed from the earth. He found his punishment there where he chose his portion, and set his heart. Every creature is to us what God makes it, a comfort or a cross, a blessing or a curse. The wickedness of the wicked brings a curse upon all they do, and all they have. Cain complains not of his sin, but of his punishment. It shows great hardness of heart to be more concerned about our sufferings than our sins. God has wise and holy ends in prolonging the lives even of very wicked men. It is in vain to inquire what was the mark set upon Cain. It was doubtless known, both as a brand of infamy on Cain, and a token from God that they should not kill him. Abel, being dead, yet speaketh. He tells the heinous guilt of murder, and warns us to stifle the first risings of wrath, and teaches us that persecution must be expected by the righteous. Also, that there is a future state, and an eternal recompence to be enjoyed, through faith in Christ and his atoning sacrifice. And he tells us the excellency of faith in the atoning sacrifice and blood of the Lamb of God. Cain slew his brother, because his own works were evil, and his brother’s righteous, 1Jo 3:12. In consequence of the enmity put between the Seed of the woman and the seed of the serpent, the war broke out, which has been waged ever since. In this war we are all concerned, none are neuter; our Captain has declared, He that is not with me is against me. Let us decidedly, yet in meekness, support the cause of truth and righteousness against Satan (Henry)
Cain, therefore, attempts to parry the question, apparently on the vain supposition that no eye, not even that of the All-seeing, was present to witness the deed. “I know not.” In the madness of his confusion he goes further. He disputes the right of the Almighty to make the demand. “Am I my brother’s keeper?” (Barnes)
Is there anything we do, (good or bad) that is hidden from God? Do we think we can keep our bad hidden from God by casting it from our thoughts? If per chance we hear the whispers for the Holy Spirit convicting us, do we deafen our ears and discard it from our minds? Do we try to argue it away by cunning logic of what our culture or society approves? Oh, that our hearts and minds were as Davids. “Search me, God, and know my heart; test me and know my concerns. See if there is any offensive way in me; lead me in the everlasting way”