Adam was in a dilemma. He wanted peace and harmony in his family, so he invoked Allah for help. Allah commanded that each son offer a sacrifice, and he whose offering was accepted would have right on his side. Abel offered his best camel, while Cain offered his worst grain. His sacrifice was not accepted by Allah because of his disobedience to his father and the insincerity in his offering.
This enraged Cain even further. Realizing that his hopes of marrying his own beautiful sister were fading, he threatened his brother: "I will kill you! I refuse to see you happy while I remain unhappy!"
Abel, feeling sorry for his brother, replied: "It would be more proper for you, my brother, to search for the cause of your unhappiness and then walk in the way of peace. Allah accepts deeds only from those who serve and fear Him, not from those who reject His commands."