All consequences have a cause.
Some other primates also feel bad when given unfair advantage. So it's not just a one off thing, it's happened multiple times. It's not just some immortal bestowing it on us.
as if anything that exists could be anything but natural.
Benevolence, righteousness, propriety and wisdom were bestowed upon us by Nature.
Rudimentary forms of morality are seen among animals, especially the "higher" primates like chimpanzees. This suggests that morality in humans is not unique and can be explained by the natural process of evolution without appeal to a divine Lawgiver.
A moral lawgiver is no more necessary for morality than a scientific lawgiver is necessary for the laws of nature.
We all agree on what morality is. We differ on the application of it.
The existence of moral laws presumes a moral lawgiver.
Well, who is the foremost source for natural development (aka Evolution)? Charles Darwin, who said that it is not intelligence, but morality that separates humanity from the other animals, and had no evolutionary explanation for morality. It serves no evolutionary purpose, and has no natural reason to have been passed down. There is no morality gene, yet it has endured.