The bellboy becomes the prime suspect in a murder when he was heard threatening the victim shortly before the murder took place.
All right, let's take care of the obvious fact first; Boris Karloff is not the killer (and I hope I haven't ruined the movie for anyone). He's that most mundane of characters for horror stars to take in comedies; the red herring. In fact, that is one of the problems with Abbott and Costello's follow-up to their meeting with Frankenstein (actually, they don't do that either; another inaccurate title); unlike that movie, which was a real horror movie on top of being a horror comedy, this one is nothing more than a by-the-numbers horror mystery comedy. Karloff does figure into my favorite scene, in which he tries unsuccessfully to hypnotize Lou Costello into committing suicide. Other than that, far too much of the comedy revolves around disappearing dead bodies, a trick that gets pretty tiresome before it's all through. However, the scenes in the caverns towards the end of the movie are certainly beautiful to look at.