Bert wants to ask which psycho he means, he's met several people who might qualify. But as they get closer to Sherlock he doesn't want to give them away by talking so he nods, even if Karkat is turned away. They can discuss potential other ways to humiliate their targets when they're done with this one.

Bert turns the knife over a few times in his hands, getting used to the weight again. It counts now, this is the time when they can't afford mistakes.