The firstborn son of Adam and Eve who murdered his brother was Cain.
According to Genesis, Cain worked as a farmer while his younger brother Abel was a shepherd. Each brought an offering to God, but God favored Abel’s offering. Cain became angry and jealous, took Abel into the field, and killed him—the first murder in the biblical narrative.
God then confronted Cain, who denied knowing where Abel was. Cain was cursed so that the ground would no longer yield its strength to him, and he became a restless wanderer. The story also introduces the “mark of Cain,” a protective sign intended to prevent others from killing him in revenge.
Cain later settled in the land of Nod and fathered Enoch. Seth, another son of Adam and Eve, was born later and is presented in Genesis as continuing the family line after Abel’s death. Cain is sometimes confused with Abel because both are sons of Adam and Eve, but Abel is the murdered brother, not the murderer.