Sherlock Season 4: 18 Easter Eggs & References From 'The Lying Detective'
9. H. H. Holmes
The other real-life explanation for Smith is referenced in the episode. Smith says his favourite historical serial killer is H. H. Holmes. He also nicked his idea of converting a building to fit his murderous needs.
Born in 1861, Herman Webster Mudgett (who went under the pseudonym H. H. Holmes) constructed a massive hotel in Chicago that locals dubbed "The Castle." Like Smith, he repeatedly fired architects and construction workers so that no one rumbled the secret alterations he made to the structure, allowing him to torture and murder his victims in various ways.
When he was later caught by the police, he confessed to 27 murders. Only nine were confirmed, but he could have been responsible for up to 200.