That one rule is Batman's, the only one he rigidly adheres to: no killing. Seems easy enough to follow until you realize what kind of pressure it causes in his line of work and how much easier it was all be if he broke it just once.
Since its mention in The Dark Knight, most publications have usually claimed that the "No Killing" rule is Batman's "Only Rule".
The Origin of Batman's “No Killing” Rule
Remember, we never kill with weapons of any kind!" This was a clear delineation that his days of stabbing criminals with swords -- like he did in Detective Comics #37 -- or giving them very public hangings were over.
Understand this, first of all — Batman did not have any rule against killing in the earliest comics. He even carried a gun and shot people to death.
By refusing to kill, Batman avoids becoming a vigilante who takes the law into his own hands. Instead, he works within the legal system to bring criminals to justice, relying on his detective skills and physical prowess to apprehend them.
The fact that he is so reliant on technology is his major weakness because he“s practically handicapped without it and could not stand his ground against a plethora of his usual enemies.
Mild spoilers for Batman: The Knight #10 aheadA lot of people think that Batman's greatest skill is fear because it's a powerful weapon against his enemies, but Batman's actual true power is compassion. Bruce may use terror to keep criminals in line, but when it comes to actually saving people, fear doesn't help.
Batman's main objective is to rid the world of such blind immorality and prevent anyone from experiencing the depraved world he was exposed to at such a young age.
Bane. The page everyone remembers from Batman: Knightfall is Bane “breaking the Bat” over his knee. But Bane beat Batman by working smarter, not harder. Bane was able to defeat the Dark Knight physically by first exhausting him spiritually (and, well, also physically).
Wayne's phobia of bats is the most apparent. It stems from his childhood incident of falling down a well with bats flying up above him. Because the incident happens at such an impressionable age, it causes extreme fear and becomes impossible to get rid of.
While Batman usually follows a strict code that prohibits lethal force, he has occasionally broken his "no-kill rule" with his enemies over the years.
The following article contains spoilers from Batman:The Knight #6, on sale now. Batman doesn't use guns. That fact has been one of the core principles of his character almost since the moment he debuted. The reason is obvious: his parents were killed with a gun, ergo he despises them and anyone who uses them.
No matter the media, the Joker remains Batman's greatest nemesis. On film, Jack Nicholson's Joker set the bar for villainy in Batman.
NUMBER 1: JOKER
Batman's greatest enemy is the Joker. The Clown Prince of Crime is a master of chaos and insanity, regularly devising complex schemes to bedevil Batman and Gotham City. His unpredictable nature and willingness to cause destruction make him an intimidating villain for the Dark Knight.
6 Lady Shiva
Shiva's unmatched skills in hand-to-hand combat have made her possibly the most feared warrior in the DC universe. Batman has never defeated her in straight combat, and she has earned a deadly reputation among other talented fighters.
“A hero can be anyone.”
Well, it's time to know how smart Batman is. A trivia published in BuzzFeed states, “Batman's stated IQ is an unbelievable 192, several notches above the famed theoretical physicist (Albert Einstein), who was estimated to have an IQ between 160 and 180.
Audiences expect you to look like a superhero.” Ironically, data collected by Moviepilot suggest that since Batman first appeared in feature-length films 50 years ago, Mr West has been the closest in physique to the Gotham crusader as specified in the comic books: 6'2" (1.88m) tall, and weighing 210 pounds (95kg).
Immense Human Healing: Batman's natural healing efficiency and speed are at the absolute highest level of human potential. He has a greatly enhanced metabolism, immune system, and vitality that runs several times faster than average humans, allowing him to heal much faster than an average human.
He is not superhuman, he doesn't have access to magic, and he bothered to give himself superpowers as well. It's estimated that Batman can lift anywhere between 450 and 1000 lbs, and with this, he is certainly above the limits of human strength, but compared to some other powerhouses, it's not impressive.
Mastor Intimidator: Batman is widley known for his unique ability to easily instill fear into others, even those who know him best fear him. He has a very high intimidation factor and are easily able to interrigate those who he needs information from in a matter of minutes.
There's no doubt that recalling the fate of his parents is Bruce's biggest fear and regret. No matter who kills Batman's parents in different media, it's the catalyst to the person he becomes. Bruce spent all his childhood and early adulthood scared of remember what had happened.
Batman worries about things like his loved ones dying, failing to save Gotham, or his successors becoming evil. Unfortunately for the Dark Knight, these fears have often manifested in reality, with Bruce Wayne having to face his worst nightmares and push his mental limits.
2012's Detective Comics #0 shows us Bruce's training with martial arts master Shihan Matsuda, from whom Bruce learns true mastery over his own body.