Nappa is not a mere mortal man. Nappa is a calamity, an apocalypse, an armageddon.
Does the tsunami that sinks islands care for what it drowns? Does the earthquake that cracks continents care for what it crumbles into debris? Does the desert that engulfs civilizations care for what it buries beneath the endless dunes?
No my friend, Nappa does not need logic, reason or motivation to do what Nappa does. There is no “why?” regarding Nappa. Nappa simply is.