Did you use your teleporter stones to get into the castle before having an invitation, and talk to Janus? If so, that is a known game breaker; the cat will not show up in the hall near the kitchen if you get the quest before officially entering the castle. There is no way around this except re-loading a save before going into the castle the first time. <img src="/ubbthreads/images/graemlins/disagree.gif" alt="" />
On the plus side, now you can kill whoever you want, without regard for consequences. <img src="/ubbthreads/images/graemlins/winkwink.gif" alt="" />