Watson was born in 1870 and worked the western states as a professional bigamist, marrying women of modest means under a series of names, taking what they had, and moving on. The number of marriages he admitted ran into the dozens.
Between 1918 and 1920, in Idaho, Washington and California, he killed at least seven of those wives rather than simply leaving them, disposing of the bodies in remote ground. He was arrested in April 1920 after a wife who had grown suspicious went to a lawyer.
He confessed to seven killings and was sentenced to life imprisonment on May 10, 1920. He died in prison on October 15, 1939.
Contemporary descriptions of him were written in the language of 1920 American newspapers, including a claim about his anatomy that the press of the day repeated at length and which has no bearing on anything. It is omitted here.
What the case is: a man who understood that a woman living alone with savings, in a period when marriage transferred control of her property, could be married, robbed and killed with very little risk of anyone asking. The convictions rest on his own confession, which is the whole of the evidence for the count of seven.