While comic villains are usually content with slaying anyone and everyone to prove their point, the heroes almost always have a "no slaying" rule. Abstaining from ending someone's life is seen as one of the major dividing lines between a hero and a villain. Even so, that doesn't mean there haven't been a few accidental — and some not so accidental — passings caused by various superheroes.
Everyone from Batman to Iron Man to Superman have accidentally slain people, with varying degrees of accident. This list of times heroes have accidentally slain people shows the accidental perishments of various characters at the hands of heroes, some of which had huge consequences in the comic universes.