So does his story: business skullduggery, a seat in Cabinet, office, great wealth then financial insecurity, family strife - all culminating in a famous suicide in Parliament buildings.
It sounds like the stuff of fiction, so why has it taken so long for a leading writer to make a novel of what the blurb on my uncorrected proof copy calls "a compelling novel of love and tragedy based on a true love story"?
The wait has been worth it. Owen Marshall calls his book The Larnachs because he chooses to tell William's tragic unravelling through the first-person narratives of third wife Conny (Constance de Bathe Brandon) and son Dougie.
It's a brilliant device, since despite Fleur Sneddon's 1997 biography and Hardwicke Knight's earlier and more speculative work 30 years ago, there remains something murky and fuzzy about the whole Larnach story.
The novel covers the last, tragic stages of Larnach's life. We meet him after he has made his pile, built his pile, buried two wives (sisters), got into Parliament, sired some generally disappointing heirs and has just courted and married his third wife, the much younger Conny.
Like any new bride, she sees the positive side of William. She sees his love for her and for his children and admires his business skills, which the real-life Larnach coupled with ruthlessness.
But things are beginning to slide.
Business, always of more interest to Larnach than politics, is starting to turn sour. The fortune he built up on a speculative bubble now looks very precarious as the Long Depression savages the value of his investments.
Conny's story hints at the trouble to come. Increasingly, Larnach is called away on business activities.
Even when he's at home, the coldness that she finds pervades the castle's rooms starts to seep into his personality.
He spends more time in Wellington or in his study and begins to absent himself from socialising with the Hockens and the rest of the Dunedin elite. And the fog that sometimes makes the journey from the castle to the city dangerous serves as a metaphor for the increasing isolation of the Larnachs.
Then, just as William draws apart from Conny, Dougie, his favourite son, draws closer to her.
It begins innocently enough with a peck of a kiss, but soon they are lovers, taking advantage of William's trips to Wellington.
Eventually, word begins to leak out, though the truth remains known only to the inner circle as yet.
The conclusion we all know. On October 12, 1898, after receiving a letter that deeply distressed him - was it blackmail? - William Larnach sat in his office in Parliament, took out a gun and blew out his brains.
While Marshall's author's note reminds us that The Larnachs is not a biography or a history, it generally follows the story of that unhappy family.
Marshall skilfully recreates the late Victorian worlds of Dunedin and Wellington and puts some flesh on the lives of Conny and Dougie, the young couple whose love was also blasted apart by that 1898 gunshot.
• Gavin McLean is a Wellington historian.
