Thor Superman Valhalla
On the point of killing and what it contributes to being a warrior, from Asgard's Realm and the other worlds in the Thor mythos, death in battle is usually a great honour, if not the greatest honour. In that same regard, giving someone "a good death" as they put it in 300, killing someone in battle, often sends them to a realm of particular importance (such as Valhalla for the Asgardians) or makes them a hero to their people, dying for the ultimate cause.
As Superman refuses to kill, he doesn't give his opponents the honourable death they seek in battle, by letting his opponents live time and time again, he continualy denies them the right to join their comrades/ancestors that have died in battle and have that "great honour" of dying in battle.
I'm almost positive nothing like that has ever been said in Comics, but I think its an interesting theory.
You might also like
Superhero Fight Free
Mobile Application (Games For Fun)
1 Set of 8 Guardians of the Galaxy Superheroes Fight Inserted Blocks