The Joker has played many cruel jokes in his time, but none were as cruel as when he manipulated the events of Jason Todd's entire life so that the boy would become a new Robin he could kill. The Joker has been a major antagonist not just for Batman but for the entire Bat-Family at large. From burning down Haly's Circus to shooting Barbara Gordon in the spine, his attacks on Bruce and his family have been varied and brutal, but few are as brutal as what he does to Jason in the New 52 canon.
Even before joining the Bat-Family Jason Todd's life was filled with trauma and tragedy. At a young age he lost his mother to drugs and his father to prison, and he was forced to survive on the streets. He finally meets Batman and becomes Robin, only to be killed by Batman's worst enemy in the pages of A Death in the Family, written by Jim Starlin with art by Jim Aparo. After he discovers that his mother didn't actually overdose, Jason leaves America and travels overseas to track her down. When he finds himself face-to-face with the Joker, the Clown Prince of Crime beats him with a crowbar and leaves him with a ticking bomb that Batman is not fast enough to stop. This is only made all the more heartbreaking when it's revealed that Joker actually planned the entire thing.
Red Hood and the Outlaws #0 by Scott Lobdell and Pasqual Ferry details how Joker orchestrated events so Jason would become Robin, with the express goal of eventually killing the Boy Wonder to hurt Batman. One day the Joker gets bored and, as with most times Joker is bored, someone's life gets ruined. Jason Todd comes to Joker's attention through his criminal father, Willis Todd, and that's when Jason's life starts to fall apart. Joker orchestrates Willis being sent away to prison, which likely isn't too hard given that he is a criminal. He also uses poison to make Jason's mother appear to overdose. With his family gone, Jason doesn't have much choice other than to turn to crime to try and survive. He avoids hurting people but he does try to steal just about everything he can get his hands on, which famously includes the Batmobile's hubcaps. Naturally, Batman isn't a fan of this, but instead of sending Jason off to juvie, he decides to adopt him and set him down another path, just as Joker wants.
Joker's entire existence is predicated on hurting Batman in whatever way he can. It's been stated before that Joker doesn't really view anyone else in the world other than himself and Batman as truly alive. As such he doesn't have any problems with killing or ruining people's lives if it gets a reaction out of Batman, just as Jason's death does. It is an incredibly twisted plan considering that even when Jason does come back to life he is antagonistic towards Bruce for everything that happens. Even the elements Joker doesn't plan for still ended up playing in his favor.
Joker has attacked or ruined almost every member of the Bat-Family in some way. Beyond infecting Duke Thomas' parents with Joker gas or nearly drowning Damian Wayne in bugs, what he does to Jason is the worst. Manipulating nearly every single aspect of Jason Todd's life, only to build up to his death for a punchline that isn't even meant for him, is by far the cruelest joke the Joker has ever pulled.