Question: "Why did God accept Abel’s offering but reject Cain’s offering? Why did Cain then kill Abel?"
Answer: The stories of the first act of worship in human history and the first murder are recorded in Genesis chapter 4. This follows the account of Adam and Eve in the Garden of Eden, their disobedience to God, and the entrance of sin into the human race. Death, the judgment pronounced upon them by God, soon made its entrance in the first family.
Cain and Abel, the sons of Adam and Eve, "in the course of time" brought offerings to the Lord (Genesis 4:3). Without doubt, they were doing this because God had revealed it to them. Some question, “How were Cain and Abel supposed to know what to sacrifice?” The answer is that God must have instructed them. It is clear that the offering was to be a substitutionary atonement, because we read in Hebrews 11:4, "By faith Abel offered God a better sacrifice than Cain did." When Abel came for worship, it was by faith that he brought his offering, the "fat portions from some of the first-born of his flock" (Genesis 4:4). The Lord looked with favor on Abel and his offering, and it was accepted.
His brother Cain brought “some of the fruits of the soil as an offering to the Lord” (Genesis 4:3). But on Cain and his offering the Lord did not look with favor. We do not know how God expressed His rejection, but it was evident. In Jude’s epistle, verse 11, we read, “They have taken the way of Cain,” referring to lawless men. This may mean that they, like Cain, disobediently devised their own ways of worship; they did not come by faith. Cain’s offering, while acceptable in his own eyes, was not acceptable to the Lord. Mostly likely, Cain’s offering was unacceptable because it was bloodless (see Leviticus 17:11); he was perverting God’s prescribed form of worship. Rather than repent at God’s rebuke, Cain became angry, and later, in the field, he killed his brother Abel (Genesis 4:8).
Why did Cain kill Abel? It was premeditated murder, caused by anger, jealousy, and pride. John wrote, "Do not be like Cain, who belonged to the evil one and murdered his brother. And why did he murder him? Because his own actions were evil and his brother’s were righteous" (1 John 3:12). The evil in his heart was further revealed when the Lord asked Cain, "Where is your brother Abel?" "I don’t know," he replied. "Am I my brother’s keeper?" (Genesis 4:9). The Lord brought a curse on Cain, and he went out from His presence.
When Jesus Christ died upon the cross, He became the substitutionary atonement for our sins. He died in our place and arose from the grave that we might have everlasting life with Him. As Abel made his sacrifice by faith, we accept Jesus’ death by faith and are made right before Him. "This righteousness from God comes through faith in Jesus Christ to all who believe." We "are justified freely by His grace through the redemption that came by Christ Jesus. God presented Him as a sacrifice of atonement, through faith in His blood" (Romans 3:22, 24).