Ledger's Joker was a very different proposition from those that had come before: In a ripped, stained suit, with clown makeup smeared across a scarred, twisted mouth, his Joker was a nihilistic, sociopathic prankster.
For example, we know that the young men who go on murderous rampages are not always sociopathic monsters but, rather, sometimes more or less "regular" men who suffered from crushing depression and suicidal ideation.