Harley Quinn has delved into a great deal of DC Universe source material, but one of the more recent stories being part of the show was genuinely shocking. Throughout the first four seasons, Harley Quinn has given incredible love to so many stories and characters from DC lore. Even when poking fun at some of the best Harley Quinn characters, the series had great affection and reverence for its source material. This contributed to one of the darkest Batman stories, based on a comic by Alan Moore, being told during season 4.

https://screenrant.com/dc-batman-the-killing-joke-harley-quinn-adapt-dark-cant-believe-op-ed/