Both Spider-Man and Batman ended up breaking their so-called one rule by unmasking in front of their loved ones' murderers - and by doing so, killed them. The two heroes are well-known for having two of the strictest no-kill rules in comics history, perhaps in response to their tragic origin stories. Both Peter Parker and Bruce Wayne are shaped by the deaths of Thomas and Martha Wayne and Uncle Ben, respectfully - and in two eerily similar stories, they both deliberately revealed themselves in front of the criminals responsible.
That both Bruce and Peter are shaped by tragic events is well-known, even among non-fans. Eight year-old Bruce Wayne lost his parents to a mugger named Joe Chill, and Peter Parker was only 15 when his uncle was killed by a man only known as the Burglar (no official name has even been specified). Both blame themselves - but only Peter was truly responsible for his uncle's death, by allowing the Burglar to pass him by when he could have easily stopped him. When Bruce and Peter received a second chance to confront Chill and the Burglar, neither hesitated.
In The Amazing Spider-Man #200, written by Marv Wolfman with art by Keith Pollard, the Burglar returns, seeking a treasure that's supposedly buried underneath Peter Parker's New York home - Uncle Ben foiled his first attempt at a break-in years ago, but that sadly resulted in Ben's death. A brutal fight ensues, ending with Peter dramatically removing his mask in front of the Burglar, who is truly terrified (rightfully so; at this point in comics continuity, Spider-Man was considered a vicious and violent menace by the entire city). Scared out of his wits and believing that Spider-Man will murder him, the Burglar suffers a fatal heart attack while running from Spider-Man and eventually collapses and dies.
Almost the exact same story beat plays out in Batman #47, written by original Batman creators Bob Kane and Bill Finger. After years of tracking down his parents' killer, Batman finally corners Joe Chill. When Batman reveals he knows Joe Chill was behind the murders of Thomas and Martha Wayne, Chill is nonplussed - how could Batman possibly prove he committed the crime? In one of the most famous Batman panels ever committed to paper, Bruce says "I know because I am the son of the man you murdered. I AM BRUCE WAYNE!!" and removes his cowl in front of Chill. Chill escapes and tells his criminal allies about the encounter...but when they realize Chill is ultimately the man responsible behind the hated Batman's existence, they collectively shoot him dead.
The final images from both stories are almost exactly the same: both Bruce and Peter hold Joe Chill and the Burglar (respectively) in their arms as they die. Vengeance led them to unmask, and vengeance ultimately led to the deaths of both criminals. This would be the last time that Peter or Bruce ever deliberately unmasked in front of a villain, but not the last time they acted out of anger or in response to their traumatic pasts. For all their differences in tone, background, and abilities, Spider-Man and Batman are more alike than perhaps either would care to admit.