Traditionally, superheroes aren’t supposed to take lives - but as it turns out, even the most responsible comic book heroes have ended the careers of their opponents in fatal fashion a time or two. In most circumstances, caped crusaders only employ lethal force when it’s absolutely necessary - to save innocent lives, typically, or for complex plot-related reasons. On rare occasions, however, even the best and brightest of the superhero community end others in cold blood.
When someone like the Punisher or Deathstroke eliminates an enemy, it’s never much of a surprise - but it carries much more emotional weight when someone like Spider-Man or Superman does it. That being said, superheroes, by their very nature, face extraordinary circumstances on a daily basis - and perhaps it shouldn’t be all that shocking that they sometimes have to cross the ultimate moral line.
When Scott Summers obliterates his mentor Charles Xavier in Avengers vs. X-Men #11, he’s under the influence of the Phoenix Force - and thus is not entirely in his own mind. However, it’s important to note that Cyclops also possesses the Phoenix Force of his own volition thanks to a zealous scheme to ignite mutant births on Earth - so he definitely still deserves at least a portion of the responsibility for Professor X’s demise.
The traumatizing incident leads to Cyclops briefly going full “Dark Phoenix” before the combined forces of the Avengers and the X-Men bring him down. Xavier eventually comes back to life and - after Cyclops's own resurrection sequence - the two end up reconciling and working together once again.
The Sentry is often described as the Marvel Universe’s equivalent to Superman - but in reality, he’s a much more complicated character than that. Robert Reynolds contains within him both the Sentry - an almighty hero who once retroactively erased himself from Marvel continuity - and the Void, an equally powerful entity bent on worldwide destruction.
Both the Sentry and the Void are manipulated into joining Norman Osborn’s ranks in Dark Reign, and that leads to them running security when he lays siege to Asgard - an ambitious plot that draws the attention of Earth’s Mightiest Heroes. Seeking to make a statement for Team Osborn in Siege #2, the Sentry picks out the biggest guy on the opposing side - Ares, God of War, and Reynold’s former Avengers teammate - and literally tears him in two without showing the slightest hint of emotion.
If there’s one hero who tries to avoid lethal force more than any other, it’s probably the Amazing Spider-Man - but even Peter Parker lets his great power get in the way of his great responsibility every now and again.
The one and only time that Spidey straight-up intentionally ends the life of an opponent, however, comes with some serious asterisks. The wallcrawler is experiencing some unexpected mutations at the time - including fangs and “stingers” on his wrists, both of which he uses to devastating effect against Morlun. Speaking of Morlun, he’s an interdimensional vampire whose entire purpose in life is to travel from universe to universe consuming spider-themed heroes - so Parker has about as much justification as he could to take Morlun’s life.
Still, it’s important to note that - despite his mutations - Spider-Man is more or less in his right mind when he makes the decision to tear out Morlun’s throat in Friendly Neighborhood Spider-Man #3 - and that’s a line he’s only crossed once.
Wonder Woman’s Lasso of Truth is generally thought of as a fairly diplomatic weapon - allowing Diana of Themyscira to have her enemies admit their wrongdoing instead of having to beat it out of them. The Lasso, however, has been used to lethal effect at least once - in an indirect sense - in Wonder Woman #21.
As part of a crossover event, that issue sees Diana faced with a rampaging, mind-controlled Superman - so she uses the Lasso of Truth to ask Maxwell Lord how to free the Man of Steel from his thrall. Lord replies that the only way is to end his life, and Wonder Woman barely hesitates before snapping Lord’s neck - saving and horrifying her Justice League teammate in the same instance.