Despite being the 'Man of Tomorrow' and an enduring symbol of hope, Superman has had his fair share of pain. Not only has his own activities caused the deaths of many civilians but he has also been unfortunate enough to witness the death of his loved ones.
From his pet dog and his parents to his love interests and his fellow superheroes, Superman has experienced a lot of tragedy that has impacted him greatly. And then in The Death of Superman, he sacrificed himself, leaving his friends and fans heartbroken. While readers will agree that each death was quite emotional, some losses proved to be more heartbreaking than others.
Krypto
Superman's Kryptonian dog Krypto has been extremely loyal to the costumed superhero for a long time. His commitment to protecting Kal-El is exemplified when he sacrifices his own life for him.
In the 1986 series Whatever Happened To The Man of Tomorrow by Alan Moore, Superman duels with the Kryptonite Man (the villain earlier known as Kryptonite Kid). As he radiates kryptonite, weakening Superman, Krypto pounces on the villain and bites off his throat. Kryptonite Man dies in a few seconds but unfortunately, Krypto gets irradiated too and eventually dies. Thankfully, the story was non-canonical and Krypto got enough storylines to make a comeback.
Lois Lane
In the Injustice storyline, the infamous Daily Planet journalist, Lois Lane, died in a heartbreaking and emotional storyline. In this storyline, the readers discover that Lois's heart is connected to a massive bomb in Metropolis. The Joker and Harley Quinn decide to kidnap her and attempt to operate on her heart to detonate this bomb.
Even though Superman arrives just in time to save Lois Lane, Joker forces him to inhale Scarecrow's fear toxin (that's also layered with Kryptonite). The hallucinatory effects of the toxin create an illusion, making Lois seem like Doomsday. Superman drags Lois towards outer space intending to kill her. It's only high above the earth that the toxin's effects wear off and he realized his folly. But it's too late and Lois dies by then.
Green Arrow
Superman's powers can be highly lethal if left unchecked or if the hero falls under undue influence. The Superman/Batman series that was initially written by Jeph Loeb featured an arc titled Absolute Power. This painted an alternate reality, where members from the 31st Century's Legion of Super-Villains annihilate most of the Justice League and raise younger versions of Superman and Batman as their own children, turning them into powerful tyrants.
The two heroes are then tasked with suppressing any vigilante who would dare to go against them. Green Arrow is one such vigilante who attempts to subjugate Superman with Kryptonite-laden arrows. He ends up having a brutal demise as Superman vaporizes him with his heat vision. 'Obey or die.' He says while decimating Green Arrow.
Jonathan Kent
Superman's adoptive father, Jonathan Kent, has died in several storylines. However, it's perhaps his death in Action Comics #870 that is the most heartbreaking as it triggers a lot of guilt in the hero.
While Brainiac attacks the Earth's sun with a missile system, Jonathan ends up having a heart attack. Martha desperately calls out for Superman hoping that he can come to her aid. Her efforts are to no avail as Superman gets engrossed in restoring the Kryptonian city of Kandor. When he returns to Earth, he is immediately filled with remorse for his inability to save Jonathan when he needed him the most.
Wonder Woman
Coming back to the Absolute Power storyline, Wonder Woman tries her best to convince a brainwashed Batman that he, along with Superman, are truthful guardians of justice in several other dimensions they exist in. Unconvinced and hell-bent on his violent ways, Batman goes on to attack her. The Amazonian warrior is left with no choice but to kill him.
Naturally, this angers Superman who wastes no time in grabbing Wonder Woman's lasso of truth and choking her with it. Needless to say, it's one of the most disturbing and graphic deaths in a Superman comic.
Batman
Joker once achieved a god-like status after tricking Mister Mxyzptlk into giving him powers in an Action Comics storyline "Emperor Joker." Christening himself as an Emperor, the villain wreaks havoc all over the world in bizarre manners.
He also seeks pleasure from torturing Batman by making the caped crusader undergo multiple deaths, as Superman looks on helplessly. One of the most distressing instances is when Joker removes his mouth and instructs predatory buzzards to feed on his flesh.
Supergirl
Supergirl's death in Crisis on Infinite Earths (immortalized by the comic-book cover of Superman holding her corpse) has been one of the most impactful deaths in DC Comics' history. The storyline mostly occurred in an anti-matter universe, where heroes like Superman and Supergirl were dragged in by Pariah.
The comic's chief villain, known as Anti-Monitor, was capable of anti-matter blasts, cosmic powers, and darkness manipulation. When Superman arrived at the Anti-Monitor's fortress (that had a solar-collector capable of destroying multiple Earths), he attacked a weaker Superman (because of his vulnerability in the altered universe). Hearing her cousin's screams, Supergirl attempted to rescue him and destroyed the solar fortress. In the chaos that ensues, the Anti-Monitor shoots a ray of anti-matter into her back, instantly killing her.
Jor-El And Lara
A recurring theme of every Superman origin story was the destruction of his home planet and the death of his parents. In the earliest variants of Action Comics, Jor-El is introduced as an experienced scientist, who tries to convince his colleagues to evacuate the planet as its core had become increasingly unstable and radioactive. But they refuse to take his concerns into consideration, leading to the planet's doom.
After Superman got to know about Jor-El and his wife Lara's fate, he has always been inspired to serve humanity considering their efforts to save him in the face of destruction.
Dr. Light
In Justice League Vol. 2 #22, Superman ends up killing Dr. Light in one of his typical 'hypnotized' displays of uncontrolled rage. When the latter accidentally attacks Wonder Woman, Superman's eyes grow red in an instant. Even before Dr. Light gets a moment to clarify his actions, Superman goes berserk and burns his head off.
Later on, it is revealed that he was actually being controlled by Atomica who had inserted kryptonite in Superman's optic nerve. Dr. Light later does return in the New 52 universe as his usual villain self but his death at the hands of Superman does show the extent to which the hero can easily be turned into a weapon of mass destruction.
Superman
No death in the DC Universe can top the death of Superman himself, at the hands of Doomsday. In the 1992 crossover event, The Death of Superman covered an intense climactic fight between the two extraterrestrial beings in visually-striking full-page panels.
In the end, both Superman and Doomsday end up mortally wounding each other. As Doomsday succumbs to his wounds, Superman, too, lays in Lois Lane's arms, breathing his last breaths. It was an extremely shocking moment for the comic-book world as Superman was largely seen as an invincible hero who could never die. Even though subsequent storylines like Reign of the Supermen resurrected the superhero, his death continues to be influential.