We can’t talk about characters who have been destroyed by the Joker without bringing up Jason Todd. In one of the most infamous stories in comics, Death in the Family, the Joker brutally beat Robin with a crowbar before finally killing him with an explosive. That
alone is pretty awful, but at least if things had ended there Jason would have simply been killed by the Joker. Instead, Jason came back and revealed how broken he was by Batman’s failure to save him.
Jason was not understanding at all about his demise. He was angry and vengeful, and took it out on everyone around him. The Joker probably could not have been happier about the way things turned out, with Jason becoming a villain and adopting the Red Hood persona. This is exactly the kind of thing the Joker aspires to achieve— corrupting a hero and stripping away their morals.