There was clearly something wrong with the kids fancy suit thing and Dexter hesitated. It almost felt like an unfair advantage and he didn't feel like he could kill the kid like that.
Perhaps he was becoming sentimental in his old age.
Or maybe the kid just amused him, especially with the heroes comment.
"Neither do I," he smiled as he ducked in close, bringing the needle up and pushing it into the kid's neck. As he injected the sedative he spoke in a low voice.
"Don't come looking for me. I'm giving you this one chance to walk away, next time I will have to kill you."
He wrapped his arm around the kid's waist as he eased him onto the ground.