As I only discovered this 3 hours before the end of the contest do not expect a really thought out answer anyway.

The Joker, The guy who is truly insane, the one who does not care about the safety of anyone, even himself. He just wants to see the world amidst chaos and destruction. To see the most virtuous men follow his footsteps into madness to be corrupt and immoral.

He is a person who would not stop at anything to prove his point that everyone is just a bad day away from being him. Thus he wants batman to cross the line the guy who became good after that one day, with only his rules keeping him from insanity.

He is unpredictable, brutal and merciless. If it comes to killing everyone in a nation man, child, elderly to make batman snap and kill him he will.

And most importantly of all he is a villain that attracts lots of audiences to batman, he is captivating and for some he is the person that keeps batman appealing.

Hope I am not too late and that this gets accounted for in the contest.