Maybe this became a load-bearing piece of Krypton continuity, I don't know.

So I think I've said more than once, I've read more about Krypton's history, than any earth history; which I kind of mean as a slam on American education rather than on my own reading habits. I also thought DC maybe went back to this well at some point, but maybe not, or at least not like this. From 1968, Superman #205, "The Man Who Destroyed Krypton!" Written by Otto Binder, art by Al Plastino. Cover by Neal Adams.

There was a Superboy story with the same title a decade prior, but this might've been a hoax, and this the real deal: Black Zero was an alien saboteur, that had appeared on earth, and when the call goes out for Superman to find him, Supes takes Chief Inspector Watkins from Scotland Yard to the Fortress of Solitude. Superman's computers find Black Zero: he was disguised as Watkins! But, it's not going to be as easy as that, as after Superman calls for the Kandorian Emergency Squad to cover the exits, Black Zero tells them--in Kryptonese--they better not, and blocks their exit with a flaming hand. He then explains, he had been on Krypton, years ago: he worked for an "interstellar pirate empire" that had him destroy worlds on the verge of space-travel. Huh, I'd have to go back through this: I thought Krypton had space-travel, then went isolationist and gave it up. Although, Superman's mom had been an astronaut...maybe I'm thinking of Zenn-La. Or post-Crisis.

Further muddying the waters of continuity, Black Zero spies on Jor-El's report to the Science Council, that Krypton was going to explode: the Council doesn't buy it, but B-Z opts to check it out. He finds Jor-El's calculations were in error, and Krypton's core was going to settle back down. B-Z then restarts the chain-reaction, doing Jor-El a solid. Now, that would've meant that if Jor-El was wrong, he launched his kid into space for no reason, and it just happened to turn out okay? The entry for him there suggests, Black Zero may have undone Green Lantern Tomar-Re's attempt to save Krypton. Black Zero was now going to blow up earth, and taunts Superman, who can't bring himself to kill him, but Black Zero walks through a wall and escapes anyway, saying he only had three days "till the fireworks!" Superman seems a bit shook, but gets some unexpected help, from the Phantom Zone! Jax-Ur could almost be considered a family friend: he had worked with Jor-El, even gone to his wedding...before blowing up Krypton's moon Wegthor in an illegal rocket accident. Actually, although that was an accident, it was also an early step in his plan to build a nuclear arsenal and take over Krypton, which explains why he seems more like a hardened criminal than a rocket scientist. (In continuity, I think that probably soured the Science Council on rockets and space travel.) Despite all that, Jax-Ur claims he wants to avenge Krypton, but he can't be trusted, especially with super-powers under Earth's yellow sun...

Jax-Ur then takes an entire page, with a flashback to Krypton, to show he can be trusted on this occasion, as he swears by "the Colossus of Hadrad!" Before the Phantom Zone was discovered, criminals arrested would be forced to take a pill, that would light them up in the "search ray." Under the ray, their skeletons would glow, even through their clothes; but one shifty criminal escaped, by exploiting a blind spot in the Colossus, a natural formation that resembled a stone face. (Really resembled; somebody had to have touched that up!) Because this was a Silver Age story, they do take pains to explain that criminal got murdered later, but the story spread, becoming an oath of loyalty to Krypton's scum. This is just a little throwaway thing, but if you think about it for more than the time it takes to read it...Krypton was messed up, wasn't it? Was the planet always a police state, and we just never noticed because ooh, flying belts! Thought-Beasts!

Superman frees Jax-Ur, who had been able to see Black Zero's hideout from the Phantom Zone. The initial battle fails, as Black Zero pulls a lot of powers out of his ass was a tricky customer, surgically augmented by his pirate bosses. After-market plastic anti-hypnotic brain shield, the ability to psychokinetically create matter, like Red or Green Kryptonite or a shield. Jax-Ur gets turned into a snake-man by said Red Kryptonite, and Black Zero gives Supes a clue where his bomb is, which turns out to be coming from space. But, it was an "anti-matter dart bomb," set to blow if it hit anything solid, and couldn't be blown early without covering earth in fallout. Superman is again stymied, and goes back to work as Clark Kent: he can't even tell his friends the end was near, figuring it would be merciful not to. But Jimmy Olsen eating a Life-Saver gives him a clue: Superman drills a hole through earth, timing it out and hitting the trajectory just right, so the dart-bomb would fly through earth and out the other side! With earth safe, Superman blows up the bomb, but it was a tough explosion even for him.

Foiled, Black Zero tries to destroy Metropolis, with a massive mental hammer, but Jax-Ur had mutated into a Medusa-like form, and turns B-Z to stone. Later, Jax-Ur smashes the statue, killing him, saying he didn't deserve the Phantom Zone. Jax-Ur himself returns to the Phantom Zone (he would later be given other shots to redeem himself, and pretty consistently fail) as Superman wonders if they would encounter the pirate empire that ordered Krypton's destruction. I don't know if that happened, but I do think other factors or agents in Krypton's destruction would come up much later. It was Galactus in the Superman/Fantastic Four crossover; a book I definitely need to dig back up before the summer...