The Killing Joke written by Alan Moore and Brian Bolland is probably the best story of the Joker ever told as it provides the readers the more sophisticated image of the Joker.
All it takes is just one bad day to drive a sane person insane.
After the Joker escapes from Arkham Asylum, he comes to Commissioner Jim Gordon and shoots his daughter Barbara in front of him, making her unable to walk afterward. Then, the Joker takes pictures of Barbara lying on the ground painfully and try to use them to drive Commissioner Gordon mad, to prove the point that “all it takes is one bad day to reduce the sanest man alive to lunacy.
In addition, the Joker also attempts to make Batman believe that he is crazy as himself, which is denied by Batman repeatedly until he accepts the last joke at the end of the story.
At one point the Joker debates his own philosophy. In his view, the Joker shows that there is not much different between him and everyone else. in the concept of "one bad day "All it takes is one bad day to reduce the sanest man alive to lunacy. That’s how far the world is from where I am. Just one bad day.”