The question of what makes someone a criminal runs throughout the thirteen stories featured in The Return of Sherlock Holmes. Are criminals born or made? Do they tend to be foreigners or can they be found next door? Why do people commit crimes? While the Sherlock Holmes stories always answer the last question at the end of each story, with a handy run-down of people's motives, the first two questions get a lot of different answers. These thirteen stories look at all sorts of crimes and all sorts of criminals. There are professional gangsters and career blackmailers; people who kill in a rage or by accident; petty thieves and stone-cold killers. Sherlock Holmes stories tend to be fairly comforting and enjoyable in that everything is always solved and answered by the end. But they can also be unsettling. Pretty much anyone can commit a crime (even Holmes and Watson themselves) or can be the victim of a crime in these tales. There's a lot of uncertainty in "criminal London," and criminal London itself isn't really confined to any one area or group in these tales.
Holmes doesn't see people as criminals or not criminals so much as he does answers to puzzles that he is solving.
Holmes's insistence on recognizing morally justifiable acts and on condemning morally unjustifiable acts means that he sees some legal criminals as not criminal, and some legally innocent people as criminal.