Mmmh, I don't agree with the commenter that think that the MC let the noble live because of compassion.
Killing that guy would solve nothing, someone similar or worst would replace him, and the same shit would happen again.
He might also get a bad reputation, or be considered a criminal, and that would seriously impede his travels, even if he's OP as F... being barred from entering a town or city would probably happen, and then what? Should he then obliterate every city that denies him entry?
Now, coercing that noble to do good by constant threat of death, that would work, I think.
What would really be awesome is seeing that douchebag change for the better, or not change at all and die as promised.