-->
Among the many meanings attributable to the noun nafs (primarily, "soul", or "mind", or "self"), there is also that of "desire" or "passionate determination" (Qamus, see also Zamakhshari's Asas); in this context, the best rendering seems to be "passion".
The innocent unselfish pleading of the younger brother had no effect, for the soul of the other was full of pride, selfishness and jealousy. He committed the murder, but in doing so, ruined his own self.