Thomas Griffiths Wainewright, once a darling of the Romantic literary movement, fell from grace after killing his mother-in-law, his uncle and his wife's half-sister.
He was arrested and tried for murder at the Old Bailey, but autopsy tests could not trace strychnine so he escaped the gallows and was transported to Van Dieman's Land (now Tasmania) where he lived out the rest of his life in disgrace.
Society rounded on Wainewright, who had mixed with Keats, Blake and other key figures in the Romantic movement, striking out virtually every record of him. All but three of his many letters have vanished and there is no sign of the diary he is rumoured to have kept.
But Motion, a Whitbread prize-winning biographer of the poet Philip Larkin and of Keats, has disclosed how he is writing a book about the gifted murderer, who was an art critic for the London Magazine and whose paintings included a portrait of Byron.
"Everyone basked in the warmth of his generous spirit, relishing his wit, encouraging his extravagance," Motion said in a lecture to the Lake District's annual Wordsworth Summer Conference.
"They just as eagerly disowned him. His paintings were scattered and lost. His collection of prints, china and drawings were sold. His wife and son emigrated to America and never contacted him again. His friends denounced him."
Though Motion concedes his man was "silver-tongued, a tremendous dandy, a compulsive liar and a forger", his obliteration now makes an objective biography almost impossible.
Motion's solution is a fictionalised confession, Wainewright the Poisoner, to be published by Faber and Faber in February.
It purports to have been written by Wainewright in Van Dieman's Land shortly before his death on the chain gang in 1847 but is annotated with facts gleaned from his few writings and literary allusions to him.
It is a precarious task for Motion. One of the few Wainewright texts he draws on is his Ticket of Leave Appeal, written in Hobart in 1844. Even this, Motion admits, is "a rag-tag of fair comments, evasions and downright lies ... as reliably unreliable as Wainewright himself."
Some facts are clear, however. Wainewright did kill - for money. Encouraged by high-powered literary friends, he lived wildly beyond his means, buying art and dining extravagantly. He forged deeds to procure property and for the money he would inherit if he killed a George Griffiths, his uncle, a Mrs Abercrombie, his mother-in-law, and Helen Abercrombie, his wife's half-sister.
Wainewright fed the Romantics' obsession that crime and high culture went hand-in-hand and the moralistic Victorians' fascination with a criminal underclass.
Charles Dickens visited him in Newgate prison and based the character, Jonas Chuzzlewit, on him. But Oscar Wilde, who also made Wainewright the subject of an essay, seems closest to Motion's view that a man should not be erased from history because he murders. Wilde wrote in the essay: "The fact that a man is a poisoner is nothing against his prose style."