فَطَوَّعَتْ لَهُۥ نَفْسُهُۥ قَتْلَ أَخِيهِ فَقَتَلَهُۥ فَأَصْبَحَ مِنَ ٱلْخَٰسِرِينَ
Commentary:
Instead of directly and briefly saying that "he killed his brother", the verse expresses the same idea in a somewhat longer sentence, viz. his mind induced him to kill his brother, so he killed him. This is done to point to the great truth that it is man’s own نفس or mind, or more properly his own "self" which often becomes the source of evil for him.