Superman's code against murder is essential to the mythos of the Man of Tomorrow. On his home world of Krypton, instead of executing criminals they were exiled from that world, residing in suspended animation in rocket ships. Later, on Earth-One, the Phantom Zone projector banished these felons to that alternate dimension. However, on two pseudo planets two Supermen ignored their illustrative past, and principals, if only momentarily.
As a side note, the silver age Superman was once tasked by the Bizarro duplicates of himself to create a variation on his own Earth-One which became Htrae. The golden age Superman encountered a world in outer space made into a carbon-copy of Earth-Two, created by one of the Evil Three Kryptonians named Mala seeking revenge on his former jailer, named "Counterfeit Earth".
Flash forward to after the Crisis on Infinite Earths, when all surviving universes were melded into one core universe. However, the Time Trapper created an artificial pocket universe which replicated Earth-One from which he spawned his own Superboy to become a sleeper agent against his foes the Legion of Super-Heroes. In this universe, three Phantom Zone criminals were also duplicated in the forms of General Zod, Quel-Ul and Zaora. After he defeated the trio by depowering them with Gold Kryptonite with the threat they would one day return seek vengeance on his own mainstream Earth, Superman used the green variation of his world's meteorite rock to forever vanquish these three threats.
Back on the pre-Crisis Earth-Two, its Superman was molded with a less lofty view of the world as he was separated from his exalted Kryptonian heritage until later in life. Hence, after once more defeating the Evil Three of Mala, Kizo and U-Ban, he did the seemingly unthinkable.
As he again outsmarted the rogues, Kal-L realized that were they to return a third time Earth-Two might not survive their sustained attack. Hence, after sealing them within a flying globe of his design, the Man of Steel lined the vessel with Kryptonite and left them in the depths of space. Years later, in Action Comics #484, the column entitled "Two Men of Tomorrow" comparing the twin Supermen states of Earth-Two's Kal-L: "The only other survivor of Krypton is his cousin Kara (Power Girl)". Ominous words indeed!
Eventually, both the Pocket Universe and that in which Earth-Two resided in ceased to exist, their history erased and with them these shameful incidents. Still they illustrate how dangerous the most powerful men in the world were when threatened by triads of terror!