The Joker that they are using is the very 1st 2 appearances of the Joker (Batman #1, 1940) in which he was portrayed as a straightforward murderer. There's a touch of the insanity from The Killing Joke, but the killer instinct is pulled from Batman #1 (1940). If you want to know what common elements in Ledger's Joker came from, then look at Batman #1 or Batman: The Man who Laughs, The Killing Joke, and then wrap it up around The Long Halloween.