Question: "Why wasn't Cain's punishment death (Genesis 4:14)?"
Answer: After Cain killed his brother Abel, God gave the following judgment to Cain: “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” (Genesis 4:11-12). It would seem to many that Cain received a lesser punishment than he deserved for murder. Why wasn’t Cain given capital punishment?
First, the punishment he received was severe. Cain believed it to be worse than death. He replied to God, “My punishment is greater than I can bear. Behold, you have driven me today away from the ground, and from your face I shall be hidden. I shall be a fugitive and a wanderer on the earth, and whoever finds me will kill me” (Genesis 4:13-14). Previously, Cain had been “a tiller of the ground” (Genesis 4:2, NKJV), so this particular punishment took away his livelihood. In addition, Cain feared death at the hands of another person. To prevent others from killing Cain, God marked him somehow (what type of mark is uncertain). Instead of being put to death, Cain was forced to live the rest of his life with unfruitful work and the guilt of having killed his brother.
Second, God had additional plans for Cain’s life. Cain’s family line is found in the verses that follow the pronouncement of his judgment. Many notable achievements are attributed to Cain’s family members (Genesis 4:20-22). On a negative note, a descendent named Lamech is mentioned as also committing murder. While the reason for this detail is not given, one possible explanation is to reveal that the judgment upon Cain extended to some of his descendants who also lived violently.
Another reason some suggest for Cain’s punishment not being death was that there were too few people on the earth. While this is one possible reason, it is not given as a clear answer in the text of Scripture. Instead, Cain had a wife (one of Adam and Eve’s other descendants) and built a town. Adam and Eve had Seth and certainly other children who provided the world’s other initial inhabitants.
Later, when God instituted the Noahic Covenant, murder became a capital crime (Genesis 9:6). The death penalty was codified in the Mosaic Law in Numbers 35:30-31, 33. Cain lived before God required death as a punishment for murder. So, God provided an appropriate punishment. The Judge of all the earth always does right (Genesis 18:25).