The article on Saki in Wikipedia states: "Politically, Munro was a Tory and somewhat reactionary in his views." It gives as a reference an essay by Dominic Hibberd in the Oxford Dictionary of National Biography. This helps clarify the meaning of the story "Dusk" and the characterization of Norman Gortsby. The young con man who sits beside Gortsby on the park bench and the elderly gentleman who "lost" the soap are "flat" characters. Only Norman is a "round" character, and the story is about his learning experience.
Gortsby is not an educated gentleman but a naive young clerical type who has probably been working all day in some office and is relaxing on a park bench before going home. He is observing the "defeated" people who only come out in the dusk. Gortsby thinks of them as
Men and women, who had fought and lost, who hid their fallen fortunes and dead hopes as far as possible from the scrutiiny of the curious. . .
When the young man sits beside him and tells his complicated story about losing his hotel, Gortsby listens skeptically. It would appear that he has no intention of giving the stranger money regardless of whether or not he is telling the truth. He says bluntly:
"Of course, the weak point of your story is that you can't produce the soap."
The young man departs muttering angrily, "I must have lost it." There is nothing sympathetic about this con man. When he sits down
As if to emphasize the fact that the world went badly with him the new-comer unburdened himself of an angry and very audible expletive as he flung himself into the seat.
He is angry when he sits down and angry when he leaves. And when Gortsby chases him down to give him a sovereign and the cake of soap:
He turned round sharply with an air of defensive hostility when he found Gortsby hailing him.
This young man wouldn't give anybody a sovereign, or even a shilling. That is part of the message.
Gortsby is portrayed as a young urbanite who thinks himself sophisticated. He has no sympathy for any of the "defeated" people he observes and none for the young man who sits down beside him. But then he finds the cake of soap on the ground and he feels ashamed of himself. He was in a bad mood and naturally was regarding the people around him with a jaundiced attitude. He feels shame, guilt, and remorse as he hurries to catch up with the young man. He is lending him the money he needs because he wants to feel better about himself. He wants to change, to atone, to be more humane, to be more Christian.
Then when he passes by the bench where he had been sitting and encounters the elderly gentleman looking for the cake of soap, Gortsby realizes he has been a sucker. That is the point of Saki's story. Saki is a Tory and a reactionary. As such, he feels that people should take care of themselves and that if they can't, then they should perish. This is a dog-eat-dog world in which the species is improved through survival of the fittest. That is the lesson Saki is teaching through the painful learning experience of his viewpoint character Norman Gortsby.
It seems possible that even the elderly gentleman is a con man who left the cake of soap by the bench intending to come back and use it as an excuse to start a conversatioin with Gortsby--but Gortsby had found the soap and had chased after the other con man. This would suggest that Saki's thesis is that you can't feel sorry for anybody and you can't trust anybody. Gortsby's experience dramatizes the reactionary message.