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.