How does the author make the landlady seem creepy in "The Landlady"?

We sense there's something odd about the landlady from the very moment poor old Billy Weaver sets foot inside her guest-house. She seems perfectly nice on the surface, but there's something more than a little eccentric about her:

“We have it all to ourselves,” she said, smiling at him over her shoulder as she led the way upstairs. “You see, it isn’t very often I have the pleasure of taking a visitor into my little nest.” The old girl is slightly dotty, Billy told himself.

But being eccentric's not necessarily the same thing as being scary. And Billy's formed the opinion of the landlady as dotty, but fundamentally harmless. However, the reader can sense something a tad more sinister in the offing. Before Billy unpacks his things, the landlady wants him to go downstairs and sign the visitors' book:

Everyone has to do that because it’s the law of the land, and we don’t want to go breaking any laws at this stage in the proceedings, do we?

At this stage of the proceedings. The implication being that laws will be broken in due course—as indeed they will be—but not just yet.
When Billy signs the guest-book, he notices that it contains only two names. Now this is when things start to get really scary. Why would there be just two names in a bed and breakfast's guest-book? Surely there'd be more than that, otherwise the landlady would've gone out of business years ago. In any case, those two names—Gregory Temple and Christopher Mulholland—do sound rather familiar to Billy. Maybe he'd once read about them in a newspaper?
Things get scarier still when he finds out that the men's names were entered into the guest-book two and three years ago respectively. Dahl is carefully building up the tension here; we know there's something wrong, but we don't know quite what. At the very least, the landlady's strange behavior gives us the creeps. And when she goes to make Billy a cup of tea, it's noticeable that she has "quick-moving hands." Now why on earth would your hands be moving quickly when you make someone a cup of tea? Unless, of course, you were putting something in it other than a sugar cube and didn't want to be seen.
When Billy finally remembers where he'd heard about the unfortunate Temple and Mulholland, and the landlady confirms that they're still in the guest-house, up there on the third floor together, we now realize that this supposedly harmless, dotty old lady is actually a murderer. But there's yet more scariness in store as Billy discovers to his horror that all of the landlady's pets are stuffed animals. As the landlady's unique brand of sugar cubes starts to take effect, Billy finally starts to put two and two together and realizes too late that he's about to become the latest exhibit in the landlady's private museum of taxidermy.