A thesis statement is generally the first few sentences of your writing to help get your readers interested in what they'll be reading. So, if your thesis is unclear, they'll be confused about what's going on before anything even happens.
Answer:the narrator is deemed unreliable due to his displays of questionable sanity. And though he makes some effort to insist he is sane, one wonders whether he is truly insane or suffers some form of psychopathy – which is not altogether the same thing.