He didn't have to because, shockingly, Bats keeps trackers in Robin's clothes. So Batman shows up and- This is where it gets hilarious. The Condiment King wanted to make Batman 'feel despair' so he wanted him to think he'd killed Robin. But like, he was too scared of what Batman would do to him if actually killed Robin, so you know what he did?

He covered him in ketchup. He gagged him and covered him in ketchup and he was pretty sure that would fool the freakin' Dark Knight into thinking Robin was dead.