i think one of the most obvious themes is that of appearances versus reality. It is interesting that the landlady is, at first at least, remarkably hospitable towards Billy. She treats him very generously and gives him his own floor for his own, and likewise charges him a minimal rent. Clearly Billy is taken in by this "kindness" which causes him to ignore the various signs that something sinister is going on here. Of course, it is all to easy to be taken in by a nice old lady who seems to be living by herself, when actually something much more sinister is happening. The fact that the tea tastes of bitter almonds indicates what is really going on, for the taste of bitter almonds is an indication that cyanide has been placed in the tea.