The Joker is the scariest of all supervillains, perhaps because he is the most creative and most versatile — really, what other supervillain can be credibly played on the big screen by Cesar Romero, Jack Nicholson, and Heath Ledger? He’s evolved over the years, from campy crook (Romero, in 1966’s Batman) to homicidal vandal (Nicholson, in 1989’s Batman) to mischievous terrorist (Ledger, in 2008’s The Dark Knight). He has no principles other than nihilism; since everything is a joke to him, he takes nothing seriously and therefore can’t be bargained or reasoned with.
There’s no convenient backstory to explain how he became the way he is (even in the Nicholson version, he was evil and sadistic from Day One). It’s as if he sprung to life from Batman’s own id, a cracked mirror image of the Caped Crusader’s own mechanical ingenuity, disregard for rules, and willingness to take the law into his own hands. Batman can pound the hell out of him, institutionalize him, even kill him, but he can’t stop the Joker from grinning that maniacal grin.
[youtube=http://www.youtube.com/watch?v=qMkkfuSizc4]