Remember, Kerb was working for those people for months. He couldn't have been a vegetable, or he probably would have gotten fired. Same thing for the other compelled servants; their minds were very heavily compelled, but they were obviously able to think enough in order to follow orders while they were away from Graendal's direct command. When the Nyneave and Elayne were compelled by Moghedien, for example, RJ blatantly stated that they were able to think and take initiative; their PoVs showed them trying to think of good answers and outdo each other. Their initiative was just focused to what they were told to do. And it has been implied throughout the story that Graendal was much more skilled than Moghedien - or anyone else for that matter. I think that it is quite possible that Kerb could have shown initiative. Especially if he was posted as a clean-up guy, to kill off the compelled victims after they had served their purpose (but like I mentioned in that thread, there really isn't much evidence for this )