According to the author, who takes advantage of the dusk?it should be a very short answer with keywords Saki's "Dusk"
Dusk, that time of day that cannot clearly be defined as either day or night that falls shortly after twilight is a time Edith Wharton called, "The moment when afternoon and evening hang balanced in midheaven." Now, with dusk, that balance tips as night nears. Yet, because it is not quite night, dusk is an ambiguous state. What better title, then, for Saki's story? For it is the time of day that those in an ambiguous state come to the park near Hyde Park in London.
As Norman Gortsby sits on a bench, with the dusk "mitigated," or lessened, by the moonlight, where many "unconsidered figures moved silently through the half-light," he ponders his own state and considers himself among what he feels are the "defeated" of dusk. That is, Gortsby feels that he, too, has failed in an ambition of his own, albeit a more "subtle ambition" than those whose fortunes have lost them money or position. At any rate, Gortsby sits on the bench taking a cynical delight in watching the passers-by and labelling them in his mind as to what kind of failure they suffer.