In the short story "Dusk," the role of light is that things can be seen much more clearly in the light. In the dimness of dusk, things are not as clear. With nightfall approaching, the defeat in man comes crawling out.
In this short story, the dimness of dusk causes the man on the bench to be skeptical, for he believes that dusk brings out the defeat in mankind. The man on the bench is skeptical of the two men that sit on the bench next to him.
While he may have been wrong about the first man's situation, he definitely was right in being skeptical of the second man's story. The second man was a true con artist who could not produce a bar of soap to collaborate his story of not knowing the name of his hotel.
When the man on the bench finds a bar of soap under the bench, he is a bit chagrined or embarrassed that he did not believe the con man's story. Chasing the con man and giving him money as the man on the bench does so is his only mistake.
When the first gentleman comes looking for his bar of soap, the man on the bench learns a lesson. He should have trusted his instincts. Dusk brings out the type of man of which to be skeptical. Of course, it is too late now for the man on the bench will never see repayment of the money he gave the con artist.
Dusk is a time when those of lesser reputation begin crawling about. It is a time to be skeptical and aware. The man on the bench fell for a con man and loses precious investment in the life of a con artist. Ironically, the man on the bench was quite aware of such a type to come crawling out at dusk.