Improper anger directed at another human is the foundation of murder. Jesus said, everyone who is angry with his brother shall be guilty before the court; and whoever says to his brother, 'You good-for-nothing,' shall be guilty before the supreme court; and whoever says, 'You fool,' shall be guilty enough to go into the fiery hell (Matthew 5:22). But Cain was not only angry with his brother, he was angry with God Himself.
but for Cain and for his offering He had no regard So Cain became very angry and his countenance fell (Genesis 4:5).Cain’s anger was directed at both Abel and God. It was directed at Abel because God had been more pleased with Abel’s offering. Cain’s anger was also directed at God because God had seen what was in his heart. God had seen that his offering was one of duty, not joy— that his offering was an act of hypocrisy, not an act of sincere thankfulness.
At this point, Cain had the opportunity to repent. He had the opportunity to confess that his offering was not given out of a deep love for God. But instead, Cain allowed God’s rejection of his offering to enrage him, thus his countenance fell. Doubtless, his heart was hardened to an even greater extent.
Then the LORD said to Cain, "Why are you angry? And why has your countenance fallen? "If you do well, will not your countenance be lifted up? And if you do not do well, sin is crouching at the door; and its desire is for you, but you must master it." (Genesis 4:6-7).God gave Cain another opportunity to repent. He told Cain that if he did well— if he ceased from making vain offerings— if he stopped kindling useless fire upon the altar of God, his countenance would be lifted up. However, if Cain failed to honor God— if he did not do well, sin would destroy him. Sin is portrayed in this verse as a beast. A beast which is enslaving if not mastered. A beast which must be conquered with good.
Cain, however, did not become a conqueror. Instead, he was conquered. He refused repentance, and his anger propelled him into a greater state of wickedness. It drove him to go straight to his brother— to tell Abel what had just happened.
Cain and Abel: Murder - Genesis 4:8
Cain and Abel: Two Routes - Genesis 4:1-2
Cain and Abel: An Offering - Genesis 4:3-5
Why Is Sin Unavoidable? (Part 1) - The Christian Worldview
How Bad Is Sin? (Part 1) - The Christian Worldview