And Cain talked with Abel his brother: and it came to pass, when they were in the field, that Cain rose up against Abel his brother, and slew him.

Wesley's Notes

4:8 And Cain talked with Abel his brother - The Chaldee paraphrast adds, that Cain, when they were in discourse, maintained there was no judgment to come, and that when Abel spoke in defence of the truth, Cain took that occasion to fall upon him. The scripture tells us the reason wherefore he slew him, because his own works were evil, and his brother's righteous; so that herein he shewed himself to be a child of the devil, as being an enemy to all righteousness. Observe, the first that dies is a saint, the first that went to the grave, went to heaven. God would secure to himself the first fruits, the first born to the dead, that first opened the womb into another world.

Jamieson-Fausset-Brown Bible Commentary

8. And Cain talked with Abel his brother-Under the guise of brotherly familiarity, he concealed his premeditated purpose till a convenient time and place occurred for the murder (1Jo 3:12; Jude 11).

Matthew Henry's Concise Commentary

4:8-15 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.