Ben Affleck’s Bruce Wayne had been operating in Gotham City as Batman for over two decades before the events of Batman v Superman: Dawn of Justice, and one of our few insights into his crimefighting career pre-meeting the Man of Steel in the DC Extended Universe came through the Robin memorial set up in the Batcave. Just like in the comics, at some point The Joker murdered the Boy Wonder, although Batman v Superman didn’t provide specific details about how Batman’s young sidekick perished at the Clown Prince of Crime’s hands. However, it was something that Snyder planned to re-explore later down the line.
During his chat with Vanity Fair about the presence of Jared Leto’s Joker in HBO Max’s Zack Snyder’s Justice League, the eponymous filmmaker talked about how he intended to show exactly how Robin died in one of his planned DC movies, along with the fallout of that horrific crime. In Zack Snyder’s words:
I’d always wanted to explore the death of Robin. And if there ever was going to be a next movie, which, of course, there probably won’t be, I wanted to do a thing where in flashbacks we learn how Robin died, how Joker killed him and burned down Wayne Manor, and that whole thing that happened between he and Bruce.
Although Zack Snyder didn’t reveal specifically which movie these flashbacks would have unfolded in, it’s worth remembering that when he boarded the DCEU, he had a five-movie arc planned. Man of Steel, Batman v Superman: Dawn of Justice and Justice League comprised the first three chapters, and the saga would have rounded out with two Justice League sequels. As such, it’s logical that Robin’s death would have been depicted in Justice League 2 or Justice League 3, although only the former was ever officially on the DCEU slate, so that’s the likelier option.
Zack Snyder also mentioned that his plan with these flashbacks was to show how The Joker murdering Robin hurt Bruce Wayne “in a way that no one has,” with this being “the most significant personal injury to his life” other than losing his parents. It doesn’t sound like Snyder believes he’ll get the chance to tell this particular story, and should the day comes that this is confirmed, hopefully he’ll inform fans about the specific beats.
That being said, Zack Snyder’s Justice League did offer the director to finally write something involving Jared Leto’s Joker, who wasn’t present in Snyder’s original vision for Justice League (Leto was among the many actors who shot new scenes in October 2020). Looking drastically different from how he was depicted in Suicide Squad, this version of the cackling villain will appear during a new Knightmare sequence, i.e. a look into a future where Darkseid has conquered Earth. Batman v Superman: Dawn of Justice provided our first look at this Apokoliptic reality, and while Snyder didn’t elaborate on how Joker fits into the picture, he described Batman’s arch-nemesis in this sequence as “sort of the Ghost of Christmas Yet To Come, supplying motivation through terror.”
Joker’s appearance in Zack Snyder’s Justice League is just one of many ways that this version of the movie will distinguish itself from the theatrical version of Justice League, with other differences including Superman donning a black suit, Harry Lennix’s General Swanwick being revealed as Martian Manhunter and Uxas/Darkseid finally showing up. Oh, and of course the fact that it’s is four hours long rather than two. While it was originally announced that the Snyder Cut would be released as a four-part miniseries, it was later decided to release it as a full-length movie.