Batman then lifts off the aircraft as the monster desperately struggles to break free from the noose, choking him to death and thus performing the world's first execution by bat-shaped plane. The Batplane then sails smoothly toward its next destination with a few hundred extra pounds of genetically altered dead weight attached, swinging like a grotesque pendulum.
Most people just hang fuzzy dice on the mirror, but whatever.
What's that? "He's probably better off this way"? Actually, Batman, he definitely would have been way better off if you'd just shared the antidote to the serum you'd created moments earlier, but only thought to use on yourself. When the scientist first injects Batman with the serum, he specifically mentions that the monsters' brains are affected by it, so they aren't in control of themselves. There's no guarantee that the cure would have worked on the mental patients, but Batman could have at least tried.
"But what if it doesn't work on them? I can't handle that kind of disappointment."