Longtime Batman fans are familiar with The Killing Joke, the 1988 one-shot comic book by Alan Moore and Brian Bolland. This dark, demented tale is considered by many to be the greatest Joker story of all time.
The Batman: Arkham video games incorporated a strikingly faithful version of The Killing Joke into their backstory, most prominently including the fateful scene between Joker and Barbara Gordon. Batman: Arkham Knight even depicted this scene in its entirety, as part of a hallucinatory flashback.
Since that young, Killing Joke version of Joker was already incorporated into the game, naturally it was only a matter of time before a crafty PC gamer — one Heitor Fraga, who goes by the handle "Tchi6" — came up with a mod that lets you play as this version of Joker. Since The Killing Joke scene was a non-interactive bit of storytelling rendered entirely via motion-capture performances, there's no "young Joker" code buried anywhere inside the game that our clever modder could use.
So Fraga made do. The mod's moveset actually belongs to the Red Hood/Jason Todd character from the Red Hood DLC pack. It's a fitting matchup, since Red Hood's moves are lethal, based on dual gunplay and the snapping of necks.
Instructions on how to mod your own PC copy of Arkham Knight to play as Killing Joke Joker can be found on this YouTube page. His channel has similar videos showing off other NPC characters that he's figured out how to mod into playability, including the Arkham Knight, Oracle (without her wheelchair), Aaron Cash and even Rocksteady developer Sefton Hill.