To answer this question, I would tell you the whole story of Cain and Abel:
- Genesis Chapter 4 contains the record of both the first murder case recorded and the first act of worship. This was after the account of Adam and his wife Eve in the Garden of Eden, how they disobeyed God, and how sin entered into the world. Death, which was God's judgment upon them then made its entrance into the first family.
- Can and Abel, Adam and Eve's first two sons, in those days went to make sacrifices to the lord God because God asked them in a revelation to do so. Some bible critics have always questioned how the two brothers knew what to offer. But bible scholars have always claimed that God must have told them what to sacrifice. It is quite obvious that the sacrifice was supposed to be for a kind of atonement, just like another part of the bible in Hebrews says that Abel, by faith offered a better sacrifice to God than what his brother Cain Offered. It was by faith that Abel brought his offering when he came to worship. Hebrews says he brought the fattest portion of some of his flock. His offering was accepted by God because God looked upon Abel and his offering.
- On the other hand, Cain his brother brought some of the fruits his farm produced as an offering to the lord. But God did not look at Cain and his offerings with favor. Though we cannot say for sure how Cain reacted when God rejected him and his offering, but it was obvious he was greatly displeased. In another part of the bible, Jude said concerning lawless men that they have followed Cain's footsteps. By this he meant that they followed the examples of Cain by devising their own style of worship due to disobedience and did not come by faith like Abel. Cain's offering was quite acceptable in his own eyes, but in God's eyes, it was quite unacceptable. This made Cain very angry, and this anger made him kill Abel his brother out there in the field.
So, why did Cain kill Abel in Bible? It was a premeditated murder, and was as a result of pride, anger, and jealousy. John said don't be like Cain, who sold himself to the devil by killing his brother. But why did he kill his brother? Because his ways were evil and his brother's ways were righteous. The evil in Cain's heart was made more evident by his answer when God asked him where his brother was. His answer was that he was not his brother's keeper. The bible recorded that Cain left God's presence after he laid a curse on him.