The first murder on the earth 

 Allah Azza wa Galla said to his prophet Muhammad (pbuh) in Surat Alma'eda ayat 27-31: "and recite to them the story of the two sons of Adam truthfully, when they both offered a sacrifice to Allah, and it was accepted from one of them, and was not accepted from the other, he said (the later) I will kill you, he said (the former) Allah accepts only from the God-fearing, if you should raise your hand against me to kill me I shall not raise my hand against you to kill you, indeed I fear Allah, lord of the worlds".

 I would rather have you earn my sin and your sin, and then you will be among the companions of the fire, and that is the recompense of the unjust, and his self permitted to him the murder of his brother, so he killed him and became among the losers.

 Then Allah sent a crow searching in the ground to show him how to hide the disgrace of his brother, he said: O woe to me, was I not even able to be like this crow and hide the body of my brother, so he stood regretful.

 These verses describes the story of Habil (Abel) and Qabil (cain), And Allah Azza wa Galla describes the evil and the consequence of transgression, envy and injustice in this story, one of them fought against the other and killed him out of envy and transgression because of the bounty that Allah gave his brother and because the sacrifice that he sincerely offered to Allah was accepted, the murdered brother earned forgiveness for his sins and was admitted to paradise, while the murderer failed and earned a losing ideal in both the lives.

 When sayyidna Adam and his wife (Hawwa) came to live in the world and started having children, it so happened that they had twins from every pregnancy, one of the two being a boy, and the other a girl, so Allah Azza wa Galla in terms of the need of the time had promulgated a special provision in the religious law given to sayyidna Adam that, as for a boy and a girl born out of one pregnancy they shall be regarded as real brothers and sisters among themselves and marriage between them shall be considered forbidden, but for boy born in the second pregnancy and a girl born in the first one shall not be legally taken as a real sister and marriage between them would be permissible. 

 But what happened was that Habil's sister was not beautiful, but Qabil's sister was beautiful, resulting in Qabil want her for himself, instead of his brother, Adam refused unless they both offer a sacrifice, and he whose his sacrifice was accepted he would marry Qabil's sister, , Habil's sacrifice was accepted while Habil's sacrifice was rejected, and thus what Allah told us about them occurred.