The Lord accepted Abel's offering but not Cain’s. This should not have been considered as a slight upon Cain, but rather as a lesson of instruction. The reason for the acceptance of Abel's offering is apparent–God desired that all offerings should recognize original sin and the necessity for a great sin-offering, the sacrifice of a life. God was beginning the lesson of instruction emphasized by the Apostle that “without the shedding of blood there is no remission of sins.” (Heb. 9:22; see also Lev 17:11) The offering of Abel was a sacrifice of a life, a true type of the promised redemptive sacrifice of our Redeemer Christ Jesus. Cain’s offering was not (it was produce) and was rejected.
“‘Why are you so angry?’ the LORD asked Cain. ‘Why do you look so dejected? You will be accepted if you do what is right. But if you refuse to do what is right, then watch out! Sin is crouching at the door, eager to control you. But you must subdue it and be its master.’” (Gen 4:6, 7) Cain was not pleasing the Lord with his wrathful and sullen attitude, so God called his attention to this. The statement that sin was crouching (lying) at the door should have warned him of the danger of a misstep. Cain failed to resist the enemy Sin, here figuratively represented as a devouring beast. The beast gained control of him – instead of Cain having the rule over it. Cain was driven to lying, unkind words and then finally to murder (“And the LORD said unto Cain, Where is Abel thy brother? And he said, I know not: Am I my brother's keeper?” Gen 4:9). Cain allowed the wrong view of matters to have control and ultimately to cause him to murder Abel. He took the wrong course in allowing envious, angry feelings against his brother to grow in his heart. He should have gone to the Lord in prayer to ask why his sacrifice was not acceptable. Cain did not learn from his experiences.
The Lord tells us that "he that hateth his brother is a murderer"–telling us also that “no murderer has eternal life abiding in him” (1 John 3:15). We are assured that the spirit of murder is the spirit or disposition of the Adversary. Satan was the first murderer (John 8:44). If we resist the devil he will flee from us (James 4:7). But if we allow anger to take possession of our minds, the Adversary comes closer and closer until he crouches at the door of our hearts. The devil becomes ready to leap in and have possession at the right moment. Cain was not counted as having sinned at the time he was angry with his brother–his sin was in the outcome of that angry mood–murder itself. We read, “Then desire when it has conceived gives birth to sin and sin when it is fully grown brings forth death." (James 1:15) Here we learn about the insidious character of sin. It does not stalk about openly in its horrible aspect, rather it starts in desire: it is prompted by selfishness of some kind, either in envy as in Cain's case or in ambition as in Satan's case.
“Cain said to the LORD, ‘My punishment is more than I can bear. Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth, and whoever finds me will kill me.’”(Gen 4:13,14) Here is where Cain became remorseful because of a guilty conscience. He mentioned regretfully that he will be hidden from God’s presence. This evidence of penitence was quickly responded to by God who graciously set a mark upon Cain so that no one finding him should slay him. Thus God guards the penitent; “A bruised reed He will not break, and a faintly burning wick He will not quench; He will faithfully bring forth justice.” (Isa 42:3; Matt 12:20) This merciful course with Cain foreshadowed God’s similar course with the whole guilty world: when his chastisements shall have brought them to repentance, then his arm will be extended for their recovery. Christ’s life was sacrificed for us and calls instead for mercy, not only upon those who slew him, but also upon the whole world. Not only was he slain by men, but he was slain for men; by his stripes all may be healed who will penitently and sincerely come unto the Father by him.