In the story "Dusk", by Saki, the young man who sits next to Gortsby at dusk in Hyde Park is someone who, supposedly, is new to London and, for that reason, has forgotten the name and location of the respectable hotel where he is currently staying. His problem is that he had gone out to get some soap but could not return to his original location because he forgot his address.
As a result, he would have to borrow money from somebody to be able to spend the night somewhere until he can get a response to a letter that he had sent his out-of-town relatives earlier. In that letter, he had given them the information as to where he would be staying during his visit.
Since the man could not produce the bar of soap that he spoke about, Gortsby immediately concludes that the young man is a liar, and duly tells him so. Yet, as Gortsby later on sees a bar of soap lying around, he realizes that the man was telling the truth, that the soap had fallen from his pocket, and that he should not only apologize, but loan him the money.
Hence, the young man's reaction of hostility is two-fold: first, because the man is indeed lying and, perhaps, is already feeling the pains of being found out. He would have never imagined that Gortsby would find a bar of soap lying around so, why would Gortsby be running after him? To hit him? To catch him and take him to the police?
The second reason why there was a look of hostility is because, truth be told, Gortsby is very annoying as a character. He is not only judgemental, but sardonic, sarcastic, quick to emit opinions and bold enough to tell people exactly what he feels- even if what he feels is wrong. It would not be a surprise that the young man had already been aggravated enough by Gortsby and his anger had probably gotten the best of him at this time. In other words, it is not very hard to be hostile to someone like Gortsby. He, unfortunately, does not prevent the common man of feeling hostile toward him, either.
The young man who tries to con Gortsby with a hard-luck story about losing his hotel appears to be a novice criminal. For one thing, he is "young," so he could not have had much experience. For another thing, he hasn't taken the obvious precaution of obtaining a cake of soap to substantiate his story if anyone should question it. He says he left his first hotel with only a shilling, so a cake of soap didn't cost very much in those days. A pourd was worth about five American dollars, and there were twenty shillings in the old British pound. So a shilling was worth about a quarter, and the young stranger supposedly had enough left over to buy a drink.
A third reason for suspecting the young man of being a novice is that he appears to be fleeing the scene when Gortsby catches up with him. He acts defensive and hostile because he has become unnerved by being exposed as a criminal by one of the first persons on whom he tried his hard-luck story. He is afraid of being arrested, and he can only think that Gortsby is going to cause him trouble, perhaps by making a citizen's arrest, or by tracking him back to his home, or by following in his footsteps and shouting for the police. The con man can't see any reason why Gortsby would come chasing after him which could be to his advantage. He is totally flabbergasted when his pursuer gives him a sovereign and a cake of soap. This experience should bolster the young novice's self-confidence greatly, making him feel that he must have a guardian angel watching over him, and taking it as a sign that he had chosen the right new career.
It seems possible that the "elderly gentleman" once tried to con the young novice, that the novice learned the entire hard-luck scam from the old timer, and that he liked it so well he decided to try it, with a few modifications, on someone else. There would be nothing so strange about this. The park could be a magnet for petty grifters after sunset. The elderly gentleman would have had a cake of soap in his pocket on that occasion, since he was an experience professional. Instead of waiting to be asked to see his cake of soap, the elderly gentleman would have planted it near the park bench and then come back to use it as a ploy to start a conversation with his potential victim.
The young con man will probably go on cheating people since his experiment with Gortsby turned out to be so profitable. If so, he will certainly carry a cake of soap in his coat pocket in the future. And he won't even have to buy one, since he got one from Gortsby as a gift.