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.
The Amazing Spider-Man #121
After abducting Gwen Stacy and the ensuing fight with Spider-Man, Green Goblin threw Stacy over the side of a bridge, causing a fatal fall. Not having enough time to swing down to her, Spider-Man shot some webbing down to grab her and the sudden stop snapped her neck.
Injustice: Gods Among Us #1
After being gassed with fear toxin laced with kryptonite by the Joker, Superman thinks Lois Lane is Doomsday. It's only after he's taken her to space and slain her that the effects wear off and he sees the terrible action he's committed.
Civil War #1
The New Warriors were looking for better ratings on their reality television show when they took on the criminals Nitro, Coldheart, Cobalt Man, and Speedfreek, but none of them were expecting how strong Nitro would be. During the battle, Nitro caused an explosion which obliterated the nearby town of Stamford, Connecticut, and led to the superhero Civil War.
Before the other Avengers can rescue him, Hank Pym single-handedly defeats the Masters of Evil and Egghead. As the other Avengers arrive and to celebrate the victory, Egghead tries to shoot Pym in the back, but has his gun barrel plugged by Hawkeye's arrow, causing it to explode and end him.