Although Superman is one of the most powerful beings in DC Comics, one battle scarred him forever, as does the fear of a particular loss. Superman’s strength and near-invulnerability ought to leave him with little to fear, but he is, at the end of the day, susceptible to the foibles and imperfections of an ordinary human, even with his unshakable moral fortitude. It’s Superman’s love for his adopted home world (and one human in particular) that further contributes to the Man of Steel’s greatest fear.
Superman #75 by Dan Jurgens and Brett Breeding is the final part of the “Death of Superman” story arc, ending with the titular hero dying as he defeats the menace known as Doomsday. Although Superman was later resurrected via the Kryptonian Regeneration Matrix, this famous storyline proved that, for all of his power, Superman could be killed. Superman’s ultimate defeat would continue to haunt him for years, as shown in the post-crisis DC Comics continuity.
In JLA Classified #14, by Warren Ellis and Jackson Guice, six members of the Justice League, including Superman, are subjected to vivid and horrific hallucinations of their greatest fears by the entity known as “Z.” For Superman, this is “the fear of being beaten to death by someone who has expressed the intent to devour your wife raw.” While this hallucination is a clear reference to Doomsday, a creature capable of physically overpowering and killing Superman, it also refers to Lois Lane dying in agony. This shows both the humanity and the selflessness of Superman simultaneously.
Superman may not be a human in a literal sense, but he was raised on Earth as a human and sees Earth as his home and humanity as the people he most closely associates with. Naturally, Superman fears death like most people, but his battle with Doomsday is especially traumatic when considering his powers. Superman is nearly invulnerable to physical attacks, so having had a being beat and stab him to death is even more unnerving, despite Superman’s eventual resurrection.
While characterized as an ordinary human, Superman is also fundamentally an altruist who thinks of others before himself, so the issue was correct to say that the death of Superman’s beloved wife, Lois Lane, is as terrifying to him as his own death. Superman may be one of the universe’s most difficult beings to kill, but as Earth’s protector, human life is precious to him, and he loves Lois Lane dearly, making the thought of her death devastating. Superman is almost impossible to kill, but his death at the hands of Doomsday forever scarred him, and the notion of his death preventing him from saving Lois Lane is truly his greatest fear.