GENESIS 4:4 - When God accepted Abel's offering, but not Cain's, Cain's "countenance fell",[1] and he attacked Abel and killed him in the field.[2]

When God confronted Cain about Abel's death, God cursed him, stating:

"What have you done? Listen! Your brother's blood cries out to me from the ground. Now you are under a curse and driven from the ground, which opened its mouth to receive your brother's blood from your hand. When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth."

As an act of irony, the curse by God focused strictly on neutralizing the benefits of Cain's primary skill, cultivating crops. When Cain complained that the curse was too strong, and that anyone who found him would kill him, God responded, "Not so; if anyone kills Cain, he will suffer vengeance seven times over," and God "put a mark on Cain so that anyone that found him would not kill him." (Which would explain why the North Vietnamese never killed him.)