Superman will destroy Batman
Everybody knows that Superman and Batman have just one rule: you don't kill. The world's finest heroes have traditionally been more than equipped to find better, non-lethal solutions to even the toughest problems. But as Man of Steel showed, sometimes that line can blur, much to the chagrin of some fans. Or more recently, in Injustice Year 2, we’ve seen Superman get hands-on with the death of Green Arrow, and even condone the whole-sale execution of Green Lanterns at the hands of Sinestro and his Corps.
While may have been outside of actual comic continuity, there have been plenty of examples of both Superman and Batman breaking their chief commandment that were in continuity - at least at the time they were published (because of that, we're leaving out books with their own continuity, like Injustice, mentioned before and to your left). Here are ten of the most prominent examples of both the Man of Steel and the Dark Knight taking a life.
Batman Kills the KGBeast (Batman vol.1 #420, 1988)
The KGBeast was a Russian super-assassin who gave Batman fits by eliminating a number of high-profile Soviet defectors in rapid succession, stymieing Batman's efforts to stop him at every turn. Batman finally caught up to the KGBeast after thwarting an attempt on the life of President Reagan.
In their final confrontation, Batman fights KGBeast to a standstill in an underground chamber. With the badly injured KGBeast challenging him to a fight to the death, and with the knowledge that the KGBeast's capture would mean extradition to the Soviets and his freedom, Batman instead pulls a cold-blooded, full-on Fortunato, walling the assassin in an underground chamber and leaving him to die, simply informing the police that they "needn't worry about him anymore."
Of course the death was retconned several years later, with Batman noting that he had the police collect the KGBeast later, but the intent of the original tale remains clear.
Superman Kills a Kryptonian Dragon (Superman vol. 3 #13, 2013)
During the "H'el on Earth" saga, in which a mysterious Kryptonian villain attempts to resurrect his long dead homeworld, Superman found himself face to face with a massive dragon of mysterious origins.
Battling the dragon furiously, Superman finally senses that the creature is in a state of cellular decay. Knowing the destruction it will inevitably cause, as it is all Superman can do to keep the creature at bay, Superman uses his heat vision on a nearby oil well to cause a massive explosion that decimates the beast.
Moments later, Supergirl arrives, informing Superman of the beast's Kryptonian nature, and setting off a chain of events leading to H'el very nearly destroying the Earth. Sure, this one isn't as monumental as others, since it's not a humanoid, and seemed to have been animated in a way outside of conventional thoughts on life, but it's notable for being the first such instance at all in the New 52.
Superman Kills Imperiex and Brainiac (Action Comics vol. 1 #782, 2001)
In the “Our Worlds At War” crossover, a living force of pure entropy known as Imperiex makes its way across the universe towards Earth, decimating and unmaking planets as they crossed his wake.
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