So the soul of the other encouraged him and made fair-seeming to him the murder of his brother; he murdered him and became one of the losers.
means, his conscience encouraged him to kill his brother by making it seem like a sensible thing to do, so he killed him, even after his brother admonished him.
Ibn Jarir said,
"When he wanted to kill his brother, he started to twist his neck. So Shaytan took an animal and placed its head on a rock, then he took another rock, then he took another rock, and smashed its head with it until he killed it while the son of Adam was looking. So he did the same thing to his brother."
Ibn Abi Hatim also recorded this.
Abdullah bin Wahb said that Abdur-Rahman bin Zayd bin Aslam said that his father said,
"Qabil held Habil by the head to kill him, so Habil laid down for him and Qabil started twisting Habil's head, not knowing how to kill him. Shaytan came to Qabil and said, `Do you want to kill him?'
He said, `Yes.'
Shaytan said, `Take that stone and throw it on his head.'
So Qabil took the stone and threw it at his brother's head and smashed his head.
Shaytan then went to Hawwa in a hurry and said to her, `O Hawwa! Qabil killed Habil.'
She asked him, `Woe to you! What does `kill' mean?'
He said, `He will no longer eat, drink or move.'
She said, `And that is death.'
He said, `Yes it is.'
So she started to weep until Adam came to her while she was weeping and said, `What is the matter with you?'
She did not answer him.
He asked her two more times, but she did not answer him. So he said, `You and your daughters will inherit the practice of weeping, while I and my sons are free of it."'
Ibn Abi Hatim recorded it.
Allah's statement,
فَأَصْبَحَ مِنَ الْخَاسِرِينَ
(And became one of the losers),
in this life and the Hereafter, and which loss is worse than this.
Imam Ahmad recorded that Abdullah bin Mas`ud said that the Messenger of Allah said,
لَاا تُقْتَلُ نَفْسٌ ظُلْمًا إِلاَّ كَانَ عَلَى ابْنِ ادَمَ الاْاَوَّلِ كِفْلٌ مِنْ دَمِهَا لاِاَنَّهُ كَانَ أَوَّلَ مَنْ سَنَّ الْقَتْل
Any soul that is unjustly killed, then the first son of Adam will carry a burden of its shedding, for he was the first to practice the crime of murder.
The Group, with the exception of Abu Dawud, also recorded this Hadith.
Ibn Jarir recorded that Abdullah bin `Amr used to say,
"The son of Adam, who killed his brother, will be the most miserable among men. There is no blood shed on earth since he killed his brother, until the Day of Resurrection, but he will carry a burden from it, for he was the first person to establish murder."
Allah said,
فَبَعَثَ اللّهُ غُرَابًا يَبْحَثُ فِي الَارْضِ لِيُرِيَهُ كَيْفَ يُوَارِي سَوْءةَ أَخِيهِ قَالَ يَا وَيْلَتَا أَعَجَزْتُ أَنْ أَكُونَ مِثْلَ هَـذَا الْغُرَابِ فَأُوَارِيَ سَوْءةَ أَخِي فَأَصْبَحَ مِنَ النَّادِمِينَ