Cain and Abel
Cain and Abel were, according to the Book of Genesis, two sons of Adam and Eve. Cain is described as a crop farmer and his younger brother Abel as a shepherd. Cain was the first human born and Abel was the first human to die.
Cain murdered his brother out of jealousy because the Lord God did not accept his sacrifice. Whereas his brother Abel offered his sacrifice from his heart in a spirit of thanksgiving to the Lord, Cain did not, as so his countenance fell. Despite the Lord God warning him of the dangers of sin, he killed his brother and was punished by being exiled.
To protect Cain from the vengeance of people, the Lord God marked him and warned that anyone who kills Cain will be punished seven-fold.