Interpretation of the climax
- Superman: I broke my oath. I killed him. Nobody has the right to kill. Not Mxyzptlk... not you... not Superman. Especially not Superman.
Mxyzptlk is far beyond Luthor, Braniac or even General Zod in that he's an Eldritch Abomination, practically an Evil God who was going to wreak havoc onto the whole universe if Superman had not stopped him, and the only way left was to put him down permanently. But Superman's inflexible Black and White Morality means Thou Shall Not Kill is absolute, no excuses, not even for an God. Since he broke his vow never to kill, he feels he can't be Superman anymore.
But... how much good might Clark still have done for the universe if he didn't stop being Superman? With great power comes great responsibility, and it's just irresponsible of him to give up all his power and abandon the universe after such an extraordinary situation where he saved the universe. Then again, he wasn't the only superhero out there, and he was basically referring to his own ego.
(Also, by insisting that "nobody has the right to kill", Superman is also being more demanding than the Trope Namer for Thou Shall Not Kill, because in the original Hebrew, God's injunction is against murder, and Superman's actions obviously fall under self-defense... however, he was reflecting on his own beliefs, and God wasn't accounting for a man who could kill people just by looking at them.)
Jordan's mocking of Superman, saying that he was deluded into thinking the world couldn't get on without him marked an end to Clark being the mask Superman wears, as John Byrne's run through to New 52 had Superman being the mask that Clark wore.
- In response to the idea that Superman surrendering his powers after killing Mxyzptlk is selfish, I think the original poster is misguided. Mxyzptlk clearly needed to be killed, and you'll notice that Superman therefore did not hesitate to do the deed. But that does not automatically absolve Superman of all guilt for Mxyzptlk's death. Superman's point here was that the concept of "Superman" is a beacon of light for all superheroes to follow. He shows what being a hero in a Fantasy Kitchen Sink means, and all other heroes look to him as an example and follow the example that he sets. And the example that he sets here is twofold: 1. heroes who kill no longer get to be called "heroes", and 2. true heroes don't let their desire to be seen as a hero prevent them from doing what needs to be done. Note that, during John Byrne's run on Superman, the Man of Steel faces a similar dilemma to that faced here by the Man of Tomorrow and fails. The Post-Crisis Superman also executes his foe(s, in this case), but because he fails to pay any price for this necessary act, he ushers in The Dark Age of Comic Books, where heroes become indistinguishable from the villains they fight, all because Superman gave them the signal that "it's okay they really have it coming". That's what the Silver Age Superman was trying to avoid. He wanted to show that the killing of an enemy is such an unconscionable act that, even when it is necessary, the hero can never recover from having walked through that door.
- This troper believes that Superman wanted to retire for the longest time, but couldn't, despite the relative lull society in general was currently enjoying, super-villain crime wise. As Lois related, Terra-Man and Parasite were dead, Brainiac was seriously disabled, and the others, including Superman's most persistent and dangerous nemesis, Lex Luthor, were lying low. But as long as Luthor was free and alive (not to mention the immensely powerful and unpredictable Bizarro and troublesome Toyman and Prankster), and Brainiac was by no means officially down for the count, he could not simply hang up his cape. But after this gauntlet was over, just about all these threats to Superman and society were eliminated, making it safe for Kal-El to finally end the "never-ending battle". Superman was definitely troubled (if not traumatized) by having to break his most cardinal rule, but ultimately, he knew it was necessary, and would do it again to save Lois, not to mention the world, from a menace as horrific as an evil Mxyzptlk. Clark saw the opportunity to leave the game and took it, using his "one rule" violation as an excuse.