After Cain killed Abel, God pronounced the sentence for his crime, which was the first crime committed since Adam and Eve ate the forbidden fruit.
We read in Genesis 4:9-15, "Then the LORD said to Cain, 'Where is Abel your brother?' He said, 'I do not know; am I my brother's keeper?' And the LORD said, 'What have you done? The voice of your brother's blood is crying to me from the ground. And now you are cursed from the ground, which has opened its mouth to receive your brother's blood from your hand. When you work the ground, it shall no longer yield to you its strength. You shall be a fugitive and a wanderer on the earth.' Cain said to the LORD, “… whoever finds me will kill me.’ Then the LORD said to him, ‘Not so! If anyone kills Cain, vengeance shall be taken on him sevenfold.’ And the LORD put a mark on Cain, lest any who found him should attack him.”
We need to keep a few things in mind as we ponder this question:
1. Cain was already sentenced to death, as are all of Adam's children. “Behold, I was brought forth in iniquity, and in sin did my mother conceive me” (Psalm 51:5).
2. God is the righteous judge (2 Timothy 4:8). We may not question God’s integrity and justice in any matter.
3. When God declared His character to Moses, He explained that He is “merciful and gracious, slow to anger, and abounding in steadfast love and faithfulness” (Exodus 34:6).
4. Only God can read the heart (1 Samuel 16:7).
5. The Law of “life for life” (Exodus 21:23) was not given to Adam and Eve, though it is a law of strict justice.
As God gave a righteous sentence, knowing what would be most fitting to Cain's heart condition, He did not allow anyone to go further than what He had determined was the best punishment for Cain.
God was a father to Adam (Luke 3:38). Those of us who are parents know that we do not punish our children to hurt them, but to teach them. God's punishments to the human race are not to get revenge on us for our sins, but to teach us. Over the centuries we are learning the consequences of following a sinful path, so that in God's Kingdom we will be glad to follow His ways, which lead to true “peace on earth.”