This seems to be, and to have been, well-nigh universal. Capture a spy and you get to torture and kill him. Capture someone who has been dropping high explosives on your cities and you are obligated (by the Geneva Convention, at any rate) to house and feed him.
I don’t get the distinction, as in why spying on the enemy is more heinous than shooting at him or blowing him up. Apparently this has been true for the greater part of human history, in that whatever mercies were shown to captured soldiers (for the most part, at least not executing them) have never been shown to captured spies. What gives? Does being spied on just piss off people more than being shot at?
Also remember throughout alot of history, spies are dedicated career professionals committed to their craft. One well placed spy could change the outcome of a war.
Soldiers in many cases are some flavor of schlub who was drafted, handed a rifle and 90 days of training, and sent out to fight. Soldiers will go home as soon as the war is over, a spy in place will continue to do damage for decades.
spies have lied in the course of their activities. Ordinary soldiers, on the other hand, (theoretically) attack without any pretense of who they are.
Spies often commit crimes against civilians and non-military targets (even if it’s just deception, theft, or other relatively minor crime (compared to killing a soldier)).
Combat (again, theoretically) occurs in designated areas (usually by the simple process of engaging in combat there). Espionage, on the other hand, strikes far from combat zones in places people expect to be safe.
Spies were historically traitors, people that had sworn loyalty (or some other oath) to the people they were spying on. If you swear an oath on your life, and later break that oath, you life is logically forfeit.
It’s also worth pointing out that for the greater part of human history, enemy soldiers were not shown any particular mercy. If they weren’t slaughtered outright, they were enslaved, or sold as slaves to some third party for a profit, or (if they were lucky) ransomed, allowing them to return home if their own people paid up to get them back. The idea that defeated enemy soldiers should be treated humanely and simply let go once the war is over is a pretty recent innovation. The question then becomes, why (even as the whole “treat defeated enemies humanely” idea was taking root) were certain activities still considered treacherous, perfidious, etc., and worthy of death.
Spies are not executed because their actions are criminal. Spies are executed to dissuade others from becoming spies. In the same way, the US among other countries impose long sentences in harsh conditions on spies as a lesson to others not to follow that path.