Laudator Temporis Acti

Personal Remembrances of Sir Frederick Pollock, Vol. II (London: Macmillan and Co., 1887), p. 237 (diary entry for February 1, 1872):

Dined Sir Thomas Watson's. Lord Chancellor and Lady Hatherley, Sir T. Henry, Boxall, George Richmond, etc. An unfortunate clergyman, bearing the same name as our host, had been tried three weeks before at the Old Bailey and convicted of murdering his wife, but sentenced to confinement for life as a lunatic. He had used the Latin words, "Saepe olim semper debere nocuit debitori," in an exculpatory statement written by him, and Sir T. Henry said that nothing had given Mr. Watson so much pain in the whole proceedings as having had his Latinity questioned. The Chancellor said that Lowe had divided the Cabinet upon it, and that he had voted in the majority affirming it to be good Latin.

On the identity of this "unfortunate clergyman" see W.P. Courtney and H.C.G. Matthew in Oxford Dictionary of National Biography:

Watson, John Selby (bap. 1804, d. 1884), scholar and murderer, baptized at Crayford church on 30 December 1804, is stated to have been the son of humble parents in Scotland. He was educated at first by his grandfather, and then at Trinity College, Dublin, where he graduated BA in 1838, being one of the gold medallists in classics, and proceeded MA in 1844. On 30 March 1854 he was admitted ad eundem at Oxford. He was ordained deacon in 1839 by the bishop of Ely, and priest in 1840 by the bishop of Bath and Wells, and from 1839 to 1841 he served the curacy of Langport in Somerset.

Watson continued his classical studies, and began a lifelong habit of writing. From 1844 he was headmaster of the Stockwell grammar school in the London suburbs. Watson continued to publish classical and other works. He translated many Latin authors for Bohn's Classical Library and wrote several biographies, including those of George Fox (1860), Richard Porson (1861), William Wallace (1861), and Bishop Warburton (1863), and Wilkes and Cobbett (1870). He wrote on the reasoning power of animals (1867) and prepared several other works, including a history of the papacy to 1530. He expected pupils to match his level of erudition. Not surprisingly, the number of pupils in his school declined and he was dismissed as headmaster in September 1870.

Watson lived from 1865 at 28 St Martin's Road, Stockwell, and there, in a fit of passion, he killed his wife on 8 October 1871. Her skull was fractured, probably by a horse-pistol in Watson's possession. She was an Irishwoman named Anne Armstrong, to whom he was married at St Mark's Church, Dublin, in January 1845. Three days after the murder he attempted to commit suicide by taking prussic acid, purchased a year earlier. He wrote a long suicide note, admitting his crime and giving instructions for the publication of his literary remnants. He claimed the loss of his post as ‘the principal cause’ of his melancholy. Watson was tried for murder at the Old Bailey and found guilty, but recommended to mercy, and the sentence was commuted to penal servitude for life. The ‘Stockwell murder’ and Watson's condition attracted much curiosity. Watson, ein unglücklicher Ehemann. Psychologische Studien über die Ehe was published at Berlin in 1875. Watson died at Parkhurst prison in the Isle of Wight on 6 July 1884 after a fall from his hammock, and was buried in Carisbrooke cemetery.

Among the sources listed by Courtney and Matthew are the following issues of The Times: January 11, 12, and 13, 1872; and July 8, 1884 (all unavailable to me).

In two letters to William Aldis Wright (January 20 and January 22, 1872), Edward FitzGerald mentions in passing the question of Watson's Latinity: see More Letters of Edward Fitzgerald (London: Macmillan and Co., Limited, 1901), p. 139, n. 1, where the editor (FitzGerald's correspondent Wright) points out that Sir Frederick Pollock misquoted Watson, whose words were actually:

Felix in omnibus fere rebus praeterquam quod ad sexum attinet femineum. Saepe olim amanti semper amare nocuit.

At any rate, this is the mark of a true scholar, to feel more shame about a possible Latin mistake than about an act of murder.

I haven't seen Beryl Bainbridge's novel based on this murder case, Watson's Apology (London: Duckworth, 1984).