In "Batman v Superman: Dawn of Justice," the film made it abundantly clear that Bruce Wayne lost someone close to him earlier in his career as Batman: Robin. Several times over the course of the movie, Batman passes by a glass case that showcases the deceased Robin's uniform, which reads, "Haha joke's on you Batman." While that clearly points a finger at the Joker, an Easter egg in "Suicide Squad" suggests he may not be the only person responsible.
Harley Quinn's dossier reveals that she was an accomplice in Robin's death. "Suicide Squad's" opening scene includes an introduction for each of Task Force X's members; each villain gets a list of traits and crimes as well as a theme song and neon-colored graphic -- and Harley Quinn's rap sheet includes Robin's murder.
Of course, that doesn't mean Harley was telling the truth when she confessed to that particular crime. As the movie goes on to prove, she's still madly in love with the Joker, which means she could have certainly lied about her involvement in Robin's murder or any number of his other crimes.
"Batman v Superman" director Zack Snyder also addressed his intention for Robin's death in an earlier interview. "In my mind, it was that Robin had died about 10 years earlier, during some run-in with a young Joker," he explained in March. Though Snyder's statement confirms the Joker killed Robin, that doesn't necessarily mean the Joker himself was the only one responsible.
The next DC Cinematic Universe installment, "Suicide Squad," stars Jared Leto as the Joker, Margot Robbie as Harley Quinn, Will Smith as Deadshot, Jai Courtney as Captain Boomerang, Cara Delevingne as Enchantress, Adewale Akinnuoye-Agbaje as Killer Croc, Karen Fukuhara as Katana, Joel Kinnaman as Rick Flag, Jay Hernandez as El Diablo and Adam Beach as Slipknot and is now in theaters.