Batman and The Joker

Seeing Batman as his equal and opposite, The Joker's seeming motive in life, beyond creating mayhem and suffering, is to convince Batman of his radical philosophy.
When Batman tries to see the inherit good in man, Joker only sees the bad. When Batman works to the bone to keep saving Gotham, Joker chaotically encourages entropy. When Batman lives by a strict code, Joker lives by no rules.
The Joker believes that because man is capable of evil, man is inherently evil, and that even "good" people are evil at the end of the day, thus making "good" people not worth saving.
In Batman, Joker sees someone aware of this philosophy but who refuses to accept it. Seeing Batman's positive influence is in direct contradiction to the Joker's belief system, so he'll do anything he can to make Batman see his way and abandon his cause.
No act, murder, torture, rape, and no victim, men, women, children, is off limits to the Joker because by the nature of his belief he views them as being just as bad as he is, and just another pawn necessary to convince Batman to his belief.
Initially trying to prove his point to Batman by convincing children to commit atrocities, when Batman refused to claim his victory by killing him, the Joker made it his quest to force Batman to kill him and thus make Batman confirm his philosophy.
Along the way, Joker had devised several plans to present a challenge to Batman, some making sense and others having no apparent reasoning.
In order to truly get underneath Batman's skin, the Joker killed his sidekick, Robin, tortured Jim Gordon, and crippled Barbara Gordon.
Misanthrope: The Joker is a classic misanthrope. He truly hates mankind and all of his crimes are motivated either by this hated or by his absurdist philosophy which stems closely to this misanthropy. This hate for man comes from his belief that mankind is inherently evil and cruel and hypocritical to believe they are capable of innocence. It is a common misconception that the Joker is a psychopath or a sadist. Neither of these are the case.
Though the Joker is a highly skilled manipulator, he is fully capable of empathy, though his thourough hatred for all mankind prevents this. He simply has the ability to twist people's emotions because he doesn't care, not because he is incapable of caring. He is not a sadist because he does not specifically get pleasure from the suffering of others. He does not tell himself people are awful to justify his actions, his actions are justified by his utter conviction that people are awful and thus their feelings are inconsequential. His violent actions against others are either a means to an end (causing Batman to suffer and ultimately break) or a whim as a result of his absurdist philosophy, which dictates that one must laugh in the face of reality to conquer it, and thus if he finds humor in the suffering of another (rather than direct pleasure) it is worthwhile. It is possible that he finds it funny that people feel pain because they cause pain. It's hard to know.
Another misconception that sometimes arises, though is interestingly close to the truth, is that Joker is not actually insane. Again, the Joker is not a psychopath, but he is, if only in a non-clinical sense, insane. Anyone whose philosophy is so utterly misanthropic and violent is mentally ill, even if such a bleak outlook comes from cripplingly severe depression. The Joker is capable of rational thought, but he is not in a right mind. Though he certainly plays up symptoms of other conditions because it makes him laugh.