[He's been credited with three during his entire career and would prefer for it to stay that way. No murder sprees, thank you.]
They were a famous murder duo in Chicago during the 1920's - both extremely intelligent and well-educated. They saw themselves as Nietzschean supermen and believed that they were capable of committing the perfect crime, which eventually led them to abducting and killing a neighbor of theirs.
Even though they were caught and only committed that one murder, their names are still fairly well known even now, almost 90 years later.