4:11 cursed from the earth. The earth had been cursed because of Adam’s sin; now the earth itself had been defiled by Cain’s sin. God’s curse was on the earth; Cain’s curse was from the earth. His boastful pride in the fruits he had been able to grow from the cursed earth had been the occasion of his sin, but now he would no longer be able to till the ground even for his own food. Those who trust in their own good works eventually find it impossible to produce them any more.
4:12 vagabond. As yet there was no law given to order man’s behavior. Therefore Cain’s crime could not be punished by governmental means, but only by its natural consequences.
4:13 punishment. The word “punishment” is usually translated “iniquity,” and its use by Cain indicates that, for the first time, Cain acknowledged his sin and guilt to the Lord. This may partially explain the degree of mercy shown by God in sparing his life after Abel’s murder.
4:14 every one. Adam had daughters as well as sons (Genesis 5:4), and brother/sister marriages were necessary at least in the first generation, before the accumulation of genetic mutations could make such close marriages genetically dangerous. Since the antediluvians lived for hundreds of years and since they could propagate children for hundreds of years (note Genesis 5:15,32), the population multiplied rapidly. This concern of Cain’s, therefore, was quite realistic. Since Cain could not produce his own food, he would have to purchase it from others, but other people would naturally tend to fear him and try to avoid him or even to do away with him.
4:15 mark. The “mark” is not described. The Hebrew word oth is better rendered “sign.” Whether this sign was a physical marking on Cain’s body or a miraculous display of some sort, it was widely known for many generations (see Genesis 4:24) and did serve to inhibit any who might be inclined to slay Cain otherwise.