Why did Cain kill Abel?
God created Adam and Eve on the same day; thus, they were the same age. Following their disobedience and Fall, God cursed the serpent, placed a curse on Eve and cursed the ground for Adam's sake [Genesis 3:14-19]. While they were still in the Garden, we are then given the cryptic words: "For Adam and his wife the Lord God made tunics of skin, and clothed them" (Genesis 3:21). "Tunics of skin" tells us that a lamb or a goat was killed and, while unstated, what else could this mean except that God instructed Adam in the matter of the atonement sacrifice? The skins were to "cover their shame." Adam thus became the priest of the family, and this has always been the birthright of the eldest son (i.e., the first-born who enjoyed the "double-portion" blessings).
The Genesis account is very brief, and the details are only revealed as we progress through the rest of Scripture. Now in exile from the Garden of Eden, Eve conceived and bore Cain, then conceived again and bore Abel [Gen. 4:1-2]. The brevity of the narrative tends to collapse the timeframe of these events in our thinking. However, while not inspired, the apocrypha claims that Adam and Eve were in the Garden for seven years while Eve was in her late sixties when Cain was born; Abel was born seven years later. The Genesis account continues: "Abel was a keeper of sheep, but Cain was a tiller of the ground and Cain brought an offering of the fruit of the ground to the Lord [while] Abel also brought of the firstlings of his flock and of their fat. And the Lord respected Abel and his offering but He did not respect Cain … and Cain was very angry" (Genesis 4:2-5).
Adam, the priest of the family, had evidently appointed the tasks for his sons. Cain, the first son, evidently had little interest in the priesthood or understanding of the atonement sacrifice, and Adam had given him the harder task working in the sweat of his face amid the thorns and thistles tilling the ground [Genesis 3:17-19]. His younger brother, Abel, was given the task of looking after the sheep for his father's sacrifices. The situation was similar to that for David [1 Samuel 17:28]. A rising resentment in Cain's mind against Abel eventually turned to murder and, when confronted by God, denial and anger that led to Cain's exile. Scripture provides the reason for the murder: "Cain who was of that wicked one, and killed his brother … because his own works were evil, and his brother's righteous" [1 John 3:12]. The "evil" in this case was a careless attitude toward God.
Following Abel's death, "Adam knew his wife again and she bore him a son and named him Seth. 'For God has appointed another seed for me instead of Abel, whom Cain killed'" (Genesis 4:25). In Adam's genealogy we are told that Adam was 130 years old when he begat Seth [Genesis 5:3] so that Cain was a "mere youth" of little more than sixty years when he was sent into exile.