The nafs (ego) as a source of evil

فَطَوَّعَتْ لَهُۥ نَفْسُهُۥ قَتْلَ أَخِيهِ فَقَتَلَهُۥ فَأَصْبَحَ مِنَ ٱلْخَـٰسِرِينَ

And his soul permitted to him the murder of his brother, so he killed him and became among the losers.