The Joker is, in some ways, a better spiritual role-model than the Batman. He is more dedicated, more effective and more efficient in advancing his cause than the Dark Knight.
The Joker isn't just bad, he's crazy bad. He is so dedicated to destruction and disorder that he inspires others to follow his lead. He embraces his true criminal roots, which are chaos, anarchy and sadism. He doesn't even care about money. Such is the zeal of the Joker's animosity that he eventually usurps Gotham's entire criminal system with a new anti-system.
The tragic irony of Batman's efforts is that he has inspired the Joker to become a particularly accomplished psychopath. Not only does the Joker have to explain this to Batman, he chastises the Dark Knight for his lack of dedication.
The Joker is far more efficient than the Batman because he inspires other people to live and die for his cause. He fans the sadistic/psychotic flames that already burn in the hearts of so many Gotham citizens. He doesn't just inspire henchmen, he inspires new leaders such as Two-Face, a man who used to be a poster boy for law enforcement.
Batman, on the other hand, works alone. He prefers to shun whatever help the police might offer, except when they stand aside and let him do things his way. At the beginning of the movie a few guys tell Batman that they're trying to follow his example, to which Batman says, "Sorry, but your equipment isn't good enough to do what I do."
Also, while Batman's cars, planes and bat-suits require the backing of a large corporation, the Joker simply uses whatever's available to any average person -- knives, guns and gasoline.
The Joker is not conflicted like the Batman, living one life during the day and then another during the night. The Joker is 100% evil, all the time, to his dying breath. He embraces his remorselessness to the point that his conscience is totally clear. He is ready to depart this life at any time knowing that he has dedicated his entire being to advancing the cause of lawlessness.
Batman, on the other hand, can't decide if he is an agent of light or dark, peace or violence, lawfulness or vigilante-ism. If he really put his mind to it, he could figure out how to work during the night and day... after all, the law is already more on his side. Yet he has earned the nickname "The Dark Knight" for sneaking around after hours as if he is ashamed of himself.
Bruce Wayne tries to focus on Harvey Dent as the new face of hope for Gotham, which is a step in the right direction... but the fact that Wayne's efforts end there demonstrates his inability to imagine infectious goodness. Notice what ends up happening: While Bruce Wayne can only think to lift up one man who's already getting the job done without his help, the Joker assembles an army of support and ends up corrupting Harvey on top of everything. Harvey would have been better off not associating with Batman at all.
Bruce Wayne fails to empower the greater populace of Gotham, while the Joker overwhelms Gotham's entire justice system by manipulating these "little people." Batman fails to understand that it's not the single man at the podium or the single man with the high-tech weapons who will change the world, it is the multitudes of average Joes (or Janes) who represent most of the human race.
Batman has a small mind for accomplishing that which is truly good. Gotham as well as the world in general doesn't need someone with super-expensive weapons or really good kung-fu skills, they need someone who is crazy good. In the same way that the Joker is a genius for evil, so also the world needs geniuses for good.
The Joker is zealously dedicated to wreaking havoc on the world; can we imagine anyone who is similarly zealous for goodness, or righteousness? Is there anyone who might inspire us the way the Joker inspires rampant crime in Gotham?
The answer is: Yes. There is one Man who is so good that He is crazy good. He is so crazy faithful and pure-hearted that He not only died for His righteous cause, but God saw fit to raise him again to demonstrate His divine approval. This crazy good Man is Jesus Christ.
In the same way that the Joker propagates evil in all kinds of unexpected and creative ways, so also Jesus Christ propagates life in unheard of, creative ways. He teaches his followers how to embrace an inconquerably hopeful perspective. He trains his disciples to aspire to a mysterious, heavenly quality of life and then go on to teach and inspire others in a similar manner.
It makes sense that Jesus should be the Great Counter-Balancer because he is the only stronghold of righteousness in the world. God has appointed him to be humanity's immovable anchor for purity and life.
Anyone who surrenders to Jesus Christ and doesn't look back will learn the secret of unraveling all the malice, greed, envy, lust and selfishness that entangles every human heart. As the Joker requires 100% dedication and trust, so also Jesus requires 100% from his followers. Batman does not understand this but the Joker does and so should anyone who understands the nature of people, or the natures of good and evil.
"When Jesus had finished saying these things, the crowds were amazed at his teaching, because he taught as one who had authority..."
-- (Matthew 7:28-29)
Batman - The Dark Knight stars Christian Bale as Batman/Bruce Wayne, Heath Ledger as the Joker, Aaron Eckhart as Harvey Dent/Two-Face, Michael Cain as Alfred, Maggie Gyllenhaal as Rachel Dawes, Gary Oldman as Commissioner James Gordon and Morgan Freeman as Lucius Fox