Spider-Man has always had a strict 'no kill' rule when it comes to taking down his enemies, but when tragedy struck the web-slinger in the worst way possible, that philosophy went straight out the widow. Much like Batman or Daredevil, some of Spider-Man's most captivating stories have explored the lines needed to be crossed in order to turn the good-hearted hero into a cold-blooded killer. In 2007, Marvel provided a blunt answer to that question in one of their most shocking storylines ever.
Fans will no doubt remember Spider-Man's Back in Black saga from writer J. Michael Straczynski and artist Ron Garney as one of the darkest tales of Peter Parker's history. After revealing his secret identity to the world in Civil War, Peter's life is capsized when the Kingpin responds to the worldwide revelation by sending a sniper to gun down Aunt May, effectively putting her into a coma. By story's end, Peter tracks the Kingpin down and beats him to a pulp, ultimately allowing him to live and instead promising he'll be back to finish the crime boss off for good the moment May passes away. Marvel would eventually drop that unresolved thread by rebooting Spidey's continuity in the highly controversial follow-up storyline, One More Day. However, while fans were left to wonder if Peter would have truly become a killer after May's passing, one of the least remembered scenes from Back in Black actually showcased Peter snapping and attempting to murder a man in a violent outrage.
The opening moments of Back in Black feature a shell-shocked Peter holding his dying Aunt May in his arms seconds after the Kingpin's sniper put a bullet in her from across the street. Filled with a rage never before shown in his eyes, Peter runs outside without hesitation, frantically repeating, "no more." The horrified look on the sniper's face says it all: Peter's completely lost his mind, and the man who just harmed his family is about to pay with his life. Within seconds, Peter grabs a Jeep from the parking lot and hurls it full speed at the sniper's perch. The sniper only barely manages to move a mere inches out of the way to avoid being killed, but there's no denying that Peter had every intention of ending the assassin's life.
However, if that isn't convincing enough to prove Peter was more than willing to become a full-blown killer in response to his Aunt's shooting, the following issue in the series cements Spidey's assured willingness to cross his most stringent moral boundary. After discovering the sniper's name, Jake Martino, and tracking the assassin-for-hire down to Grand Central Terminal, Peter breaks Martino's arm and smashes his face into cement before promising to end his life quickly if he simply fesses up to who hired him. Before Martino can respond, another Kingpin-hired assassin silences Martino by shooting and killing him. Later, Peter takes a moment to ask himself if he was honestly ready to kill Martino. Without shame, he admits that he was.
Back in Black is a heavy storyline that tragically showcases Spider-Man on the brink of insanity, ready to cross a moral line he would have never returned from. Interestingly enough, Peter somehow manages to hold back just enough while beating on the Kingpin, but when it came to the man who actually pulled the trigger that shot Aunt May, Peter had no tolerance left within him. Even sadder, Peter's unhinged anger and willingness to commit murder stems directly from his own guilt. In classic Spider-Man fashion, Peter unmasked himself to the world with the thought that it was the most responsible choice at the time, only for that decision to directly result in his Aunt being shot - a repeating cycle Spider-Man has tragically been unable to escape since the death of his Uncle as a consequence of his own selfish motives.
