Well that version of the Joker was a study in the duality between Batman and the Joker. With Batman having been off the scene for some time, Joker no longer had any reason to exist. He's always seen himself as the yin to Batman's yang. With no Batman, there was no need for Joker. That's why when Batman disappeared, Joker reverted back to some semblance of sanity. With Batman's reemergence, the Joker also reemerged. However, he was still much saner than before, hence the less flamboyant clothing and more calculating attitude. He also knew that with Batman's more aggressive, take-no-prisoners style, his only shot at bringing down the Bat was to frame him for his murder. That's why he killed himself. In his own twisted mind, he had already accomplished what he wanted all along... to turn Batman from hero to villain. Killing himself was just a means of speeding up the process. In essence, Joker won that fight, even though it cost him his life. That, to me, is what made that Joker scarier than the Clown Prince of Crime Joker.