Batman can't live without the Joker. The Joker can't live without Batman. Moment of epiphany for the Bat: Maybe the world is better off without the both of them.
Acid green eyes met icy blue ones. "Oh, Batsss. Must you rain on my parade?"
"You're going back to Arkham Asylum, Joker." Batman said stoically.
"Hm-mhh...I just love it when ya try to force-feed me justice. It's too cute." The Joker straightened his suit jacket and stuck his tongue out, waggling it a little. "Just...try to smile more, woncha?" He took a smoke grenade out, popping the pin with great relish. "See...I don't want to go back to Arkham. Much as I miss my fellow inmates!"
Batman scowled as smoke filled the narrow street alley, climbing up the dilapidated walls of the buildings. "No more games, Joker! I'm not here to play Hide and Seek!"
He shot his grappling hook up at the rooftop and scaled up to gain a better view of the situation. Down below, the smoke was getting thicker, if that was even possible. Straining his eyes to get a glimpse of a thin, wild-haired silhouette, Batman nearly missed the chuckle at his ear.
Spinning, he managed to snag the Joker's jacket and throw him backwards. "That's it, Joker. That's it! That explosion killed hundreds at the hospital. Barbara Gordon was one of them. Is that your idea of fun?" Batman drove his fist into the Joker's gut with all his strength, expelling the air from the the Joker's lungs. Atop the rooftop, he straddled the Prince of Crime and struck out violently, landing punches at every inch of the clown. "Goddamnit, Joker! Is that your idea of fun?"
The Joker tipped his head back, blood trickling down his chin. He laughed.
Laughed, long and loud. Laughed right in Batman's face, eyes crinkled in mirth.
Batman stared back, stunned. For the first time, it fully registered: the Joker was lost. He was insane, and nothing could ever help him or rehabilitate him. There was no one here to save; this was an empty, hollow shell of a maniac, and goddamnit, the Joker was lost. His muscles lost their strength all of a sudden.
The Joker could have shoved him off in that moment of weakness, but he didn't. "Look at you, Bats! Look at you! A dark, vengeful king. A god."
"I'm not a god." Batman said bitterly. "Would that I were."
"Everything I do – everything we do – we do for you. Sacrifices for this angry bat-god...sacrifices to make you stronger. Oh, Batsss...we in Arkham Asylum...we love you sooo much. We-"
"Shut up!" The words came out as a low growl. "Don't lie to me, Joker. I know your mind games: so now all those deaths were my fault? Don't tell me you kill because of me! Don't you dare-" Batman gritted his teeth, feeling his jaw clench. "And if I'm your god, then what are you? The Devil?"
"Ahhh...you could say that, darling. But what you don't know...what the Bible doesn't say...is that the Devil is God's friend. Constantly challenging Him...giving Him obstacles, making Him more powerful." The Joker licked his lips languidly. "It is because of the Devil that people need God! Can't you see?"
Batman shook his head numbly, sick to his stomach. He wondered if Comissioner Gordon had received the news of his daughter's death yet. As if being shot in the spine and paralysed wasn't enough. "I wish I could kill you." He said hoarsely.
"You wouldn't." The Joker chuckled sagely. "You couldn't. We're two sides of the same coin. One cannot exist without the other. Batman can't be Batman without me!"
Batman was silent.
"What is it, Batsy-poo? Brooding?" The Joker screamed with laughter.
"I hear you." Batman's voice was odd, almost tinny, as he regarded the Joker with empty blue eyes. "You know what?"
"What, Bats? I'm all ears!"
"The world will be better off without us." Batman whispered. He gripped the Joker with a tight, vice-like grip, and flung the both of them off the rooftop.
And so the Bat-God held the Devil to his chest, and made the final sacrifice.
They say the Devil laughed all the way down to hell.