When you think about the relationship between famous heroes and their biggest rivals a question naturally emerges: what happens when our favourite superheroes get pushed too far?
Because where villains can kill someone just for the sake of it, for heroes this matter is far more complicated. With the majority of costumed crusaders having moral codes and limits to restrict them, it takes just that little bit extra for them to actually murder someone - even if that someone is their most hated rival.
Generally, this means any comic containing a murderous hero also contains some epic slight against them, that finally prompts them to take that route they've avoided for so long, and finally crush those who have conspired harm against them.
Or, alternatively, you could be Moon Knight, and just rip a man's face off without any sort of ethical quandary - although even that comes with its own special set of ramifications.
While these deaths often last for a hilariously short amount of time, they still always have a profound impact on the story, either by effecting the hero, or by depriving a given comic of a longtime adversary.
10. Superman Kills Lex Luthor - Injustice: Ground Zero
With Luthor having killed Superman a fair few times in various series and spin-offs featuring the pair, it seems only fair that the Man of Steel also gets to murder Luthor at least once. That 'once' comes in Injustice: Ground Zero, and is exactly as brutal as the rest of the series.
After the pair had been allies for years, Luthor finally realises that his Kryptonian friend has pretty much become Earth's next big dictator, and begins to work against him. In issue #19, the scientist decides enough is enough, and decides to blast the decidedly unheroic superhero from the sky with a Kryptonite ray.
This fails, however, as Lex pauses for a split second - heartbroken at having to destroy a man he had begun to genuinely consider a friend.
In that moment, Superman becomes aware of his intentions, and body slams the unfortunate bald man before snapping his neck with one hand.