Even if Vegeta spares him, he probably just leaves him there to die of his injuries and goes to fight Goku.
Later, when Vegeta creates the artificial moon, Nappa transforms too. After that, there are three scenarios.
1. Nappa is still too injured to move.
2. Nappa's injuries do not affect his ape form, but he loses his reason when transformed.
3. Nappa's injuries don't affect his ape form, he keeps his reason and helps Vegeta against the Z-Fighters.
4. Nappa's injuries don't affect his ape form, he keeps his reason, and turns against Vegeta to take revenge on him for leaving him behind.
I'll analyze each of those scenarios later. Stay tuned.