Faced with a seemingly Joker-ized Batman, Damian has no choice but to defend himself from his father's attacks, after Joker reveals that this strand of his toxin is open to commands, such as "Kill Robin." Damian holds back, trying to reach his father somewhere under the laugh, but it's no use as Batman goes straight for the kill, with every single attack, forcing Damian to fight back.
Of course, during the entire fight, Joker is crackin' wise the whole time, taking great pleasure in watching the demented situation he has authored. Damian, really not wanting to fight his father, manages to escape, farther into the zoo, but ends up being knocked into the water by Batman, as Joker watches on (making Aquaman cracks) from the tank windows below.
Damian is finally pushed to his limit, and starts fighting back for real this time. As he grabs a jagged icicle, with intent on using it to do some serious damage, he has second thoughts, dropping it, proclaiming he'd rather die then do what the Joker ask of him... Then Batman's chest explodes with a burst of gas, as that's not how Joker wanted the situation to play out. As Damian succumbs to Joker's gas, with his last bit of strength, takes one of Joker's screwdrivers, and stabs it through Joker's foot. In a scream of pain turned to laugh, Joker begins to scream, drawing a mustache on Damian's face, with his own screwdriver blood. Joker also reveals that Batman was actually a dojo master who owed some favors, to some bad people, and that was that.
As with everyone, Damian wakes up to Joker and his silver platter...