Abel was intelligent, obedient, and always ready to obey the will of Allah. This contrasted sharply with his brother, who was arrogant, selfish and disobedient to his Lord. Abel did not fear his brother's threats, but neither did he want his brother to be hurt. Allah had blessed Abel with purity and compassion. Hoping to allay the hatred seething in his brother, Abel said: "My brother, you are deviating from the right path and are sinful in your decisions. It is better that you repent to Allah and forget about your foolish threat. But if you do not, then I will leave the matter in the hands of Allah. You alone will bear the consequence of your sin, for the Fire is the reward of the wrong-doers."

This brotherly plea did nothing to lessen the hatred in Cain's heart, nor did he show fear of Allah's punishment. Even familial considerations were cast aside. Cain struck his brother with a stone, killing him instantly. This was the first death and the first criminal act committed by man on earth.

When Abel had not appeared for some time, Adam began to search for him but found no trace of his beloved son. He asked Cain about Abel's whereabouts. Cain insolently replied that he was not his brother's keeper nor his protector. From these words his father understood that Abel was dead and Adam was filled with grief.

The First Burial
Meanwhile, Cain did not know what to do with his brother's corpse. He carried it on his back wandering from place to place, trying to hide it. His anger had now subsided, and his conscience was saddled with guilt. He was tiring under the burden of the corpse which had started to have a stench. As a mercy, and to show that dignity could be retained even in death, Allah sent two ravens that began fighting, causing the death of one. The victorious bird used its beak and claws to dig a hole in the ground, rolled its victim into it and covered it with sand.
Witnessing this, Cain was overcome with shame and remorse. "Woe unto me!" he exclaimed. "I was unable to do what this raven has done, that is to hide my brother's corpse." Cain then buried his brother. This was also the first burial of man.

The First Murder - from Quran
Allah the Almighty revealed: And (0 Muhammad) recite to them (the Jews) the story of the two sons of Adam Abel and Cain - Habil and Qabil respectively in truth; when each offered a sacrifice (to Allah), it was accepted from the one but not from the other. The latter said to the former: "I will surely kill you."
The former said: "Verily, Allah accepts only from those who are Al-Muttaqeen (the pious). If you do stretch your hand against me to kill me, I shall never stretch my hand against you to kill you, for I fear Allah; the Lord of the Alamin (mankind, jinn, and all that exists). Verily I intend to let you draw my sin on yourself as well as yours, then you will be one of the dwellers of the Fire, and that is the recompense of the Zalimeen (polyt heists, and wrong-doers)."
So the Nafs (self) of the other (latter one) encouraged him and made fair-seeming to him the murder of his brother; he murdered him and became one of the losers. Then Allah sent a crow who scratched the ground to show him to hide the dead body of his brother. He (the murderer) said: "Woe to me! Am I not even able to be as this crow and to hide the dead body of my brother?" Then he became one of those who regretted. Surah 5:27-31

The First Murder - from Various Hadith
Ibn Abbas, Ibn Masud and a group of the companions of the Prophet related that inter-marriage of the male of one pregnancy with the female of another had been in practice among Adam's children. Abel wanted to marry Cain's sister, but Cain wanted her for himself because she was very beautiful. Adam ordered him to give her in marriage to his brother, but he refused. So Adam ordered both of them to offer a sacrifice, then went to Mecca (Makkah) to perform the pilgrimage. After Adam had left, they offered their sacrifices: Abel offered a fat lamb - he was a shepherd - while Cain offered a bundle of the worst grain. A fire descended and devoured Abel's sacrifice, leaving that of Cain, so he became angry and said: "I will surely kill you so that you will not marry my sister." Abel replied: "Allah accepts from those who fear Him."
According to Abu Jaafar Al-Baqer, Adam was watching their offering and was sure that Abel's sacrifice would be accepted. Cain complained to Adam that the acceptance was due to his supplication for Abel and that he had not done the same for him, so he promised his father to settle the matter between himself and his brother. One night, Abel was late returning from tending his flock. Adam sent Cain to see what had happened to him. When he found him, he glared at him saying: "Yours was accepted, and mine was not." Abel replied: "Allah only accepts from the Allah-fearing." Cain became angry on hearing this and hit him with a piece of iron that was with him, thus killing him. In another version it was said that he killed him with a rock to the head while he was sleeping.