Analysis: Whatever Happened to The Man of Tomorrow?

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.

The climax of the story is Superman being forced to kill Mr. Mxyzptlk, and since he violated Thou Shall Not Kill he gives up his powers. This is either his noblest act... or his most selfish.

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.

By this, Alan Moore was saying how the simplistic morality of the Silver Age is not enough for a more realistic world, a theme he would explore in Watchmen.

(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.