Zack Snyder’s Batman v Superman: Dawn of Justice isn't afraid to lean into the darkness. In fact, the battle between the two titans was fierce, including things like Batman’s mech suit and kryptonite spear, and Superman’s brawl with Doomsday was equally ferocious. Nevertheless, the most brutal part of the film had nothing to do with Batman’s conflict with Superman -- it was all about the Dark Knight’s backstory.
Batman is typically portrayed as a heroic vigilante who fights, within his principles, for the good of his city. One of the rules that most iterations of the Dark Knight follow is that he will not kill his foes, but that changed with Snyder’s version of the character, and two specific elements made this disturbing turn all the more terrifying.
The Bat Brand
Ben Affleck's version of Batman has been a vigilante for twenty years, which means that he is nearly fifty years old at the time of the film. Everything he has been through and everyone he has fought against has changed his perspective, and he's become a hardened fighter with a bleak outlook on his line of work. And with that being the case, he employed some new rules for protecting Gotham from its worst criminals.
Early in the movie, Batman is seen branding Cesar Santos. It's a barbaric practice that underscores the hero's new, bleak worldview, and Santos wasn't his only victim. Eighteen confirmed criminals that bore the mark of the Bat on their person, and in the Batman v Superman: Dawn of Justice Tech Manual, props master Doug Harlocker explained that by branding people, Batman was taking the necessary steps to grow the fear behind his brutal persona. "By permanently marking the worse of the worst, it's a warning to those out there of what they're dealing with," he explained.
By employing this practice, Batman was coming awfully close to crossing his most important rule, as the mark equated to a death sentence in prison. There was no way to hide the mark, so when inmates saw the brand, many would act as executioner because they knew that the Dark Knight only branded the most despicable criminals. In other words, Batman may not be pulling the trigger, but metaphorically, he was loading the gun and cocking it for someone else to punish and kill Gotham’s worst. The idea was even thrown around to have Batman brand Lex Luthor, which would've dramatically changed the tone of the movie's conclusion.
Robin’s Broken Weapon
One of the many shocking parts of Batman v Superman was finding out about Robin’s death. The circumstances of the Boy Wonder's demise aren't stated in the film, but the suit with the Joker’s spray paint on it speaks volumes. The tech manual provides some context when it explains that "Robin's desecrated suit stands as a reminder for Batman of just what's at stake when he fails to get the better of the criminals he spends his life trying to bring to justice."
The events that surround Robin’s death clearly contributed to Batman’s bleak outlook, but his dark turn started even before that, and the weapon in the Robin memorial is proof of that. The suit holds a broken halberd, which is a combination of a battle-ax and a spear that was popularized in the medieval ages as one of the most fearsome and lethal weapons of its time.
Nightwing has used batons in some comics, and Tim Drake has employed staff before, but Robin’s using a halberd proves he must've adopted the same brutal outlook as Batman. The fact that the weapon was broken is also important, as it's unlikely that was always the case. Doug Harlocker explained that the broken weapon "was Zack [Snyder] saying ‘look, maybe we should just make it look as if it's been broken and smashed.'" Using the weapon, the filmmakers were telling the audience that everything Batman had been fighting for twenty years was getting the best of him. His worldview had been smashed, and, as the end of the movie proved, it wasn't easy to get through to him.