R. Austin Freeman 
The Uttermost Farthing [EPUB ebook] 

In this tightly plotted novel, we follow the bizarre career of a man who loathes criminals. After Humphrey Challoner finds his wife killed by the bullet of a robber in his home, he vows to catch the man. Mr. Challoner is a wealthy savant, and he saved the fingerprints of the robber as well as some of the robber’s strange hair which Challoner’s wife had in her hand. For twenty years Humphrey hunts for the killer, in the process revenging himself on London’s criminals class in a surprising and gruesome fashion. This Freeman work is not a Dr. Thorndyke mystery. Instead, it is the disturbing account of how a man wreaks vengeance on the miscreant who murdered his wife in a bungled robbery attempt. It also poses the question of how a moral society should deal with criminals.