Then the LORD said to Cain, "Where is Abel your brother?" (Genesis 4:9)Prior to the murder, Cain deserved death (see All Men Are Guilty). Cain was kept alive only because of the mercy of God. He rejected God... and he remained alive. He did not seek to give his best offering... and he remained alive. He grew in anger... and he remained alive. But then Cain murdered— he took the life of his very own brother. It seems that God should have come in immediate wrath.
Surprisingly, God simply asks Cain a question. A question which God could have certainly answered Himself. Why then did God ask the question? Perhaps God was giving Cain another opportunity to repent. But instead of repenting, Cain followed his recent murder with a lie:
And he said, "I do not know. Am I my brother's keeper?" (Genesis 4:9)Cain answered God's question with a lie... and then he asked God a question— a question which was brimming with unrighteous sarcasm. Notice how easily Cain speaks of his brother who he just murdered. Cain was devoid of love and was able to sarcastically regard the entire situation. Although it is nothing more than a possibility, perhaps when Cain murdered his brother, he thought, "God wants a sacrifice? Okay then— I'll give Him the sacrifice He wants... I'll slaughter Abel!"
Not only does Cain fail to answer God's question in an appropriate way, but he also fails to confess his murderous crime. He fails to turn to God in repentance, and so God replies to Cain with a curse.
Cain and Abel: Cursed - Genesis 4:10-12
Cain and Abel: Murder - Genesis 4:8
Why Is Sin Unavoidable? (Part 1) - The Christian Worldview
How Bad Is Sin? (Part 1) - The Christian Worldview
The Fall: No Repentance - The Christian Worldview