According to mystics, magicians, witches, warlocks, wizards and nearly all other practitioners of magick ... according to Aleister Crowley and according to Grant "Pop Magic!" Morrison ...
Laughter is the best means for banishing a demon.
So with the red and black rose petals, was Joker giving Bruce a weapon to use against the demon? Bruce was "at the end" where he was supposed to be broken and potentially succumb to the devil's psychic possession or attack? Was the Black Glove merely "the modern incarnation of the occult", servants of the demon Barbatos, doing his will, offering the sacrifice of the Wayne family member to the Bat-Demon?
The demon wants Batman. By making him laugh, (Was it chemically? We're to understand that Bruce had an antidote ... so why the laughter? Like I said a second ago ... Bruce is now in on the joke.) did Joker give Bruce the power to banish Barbatos from his mind? (This is why when we next see him, Batman is thinking clearly, ready to kick ass and take names. And Hurt - channeling Barbatos' words, probably - "curses" him.