Herbert Rowse Armstrong was born on 13 May 1869 in Plymouth, Devon, England. He pursued his education at St Catharine's College, Cambridge, earning a BA in Law in 1892 and later an MA in 1901. After qualifying as a solicitor in 1895, Armstrong initially practiced in Liverpool and Newton Abbot before relocating to Hay-on-Wye in 1906. In 1907, he married Katharine Mary Friend, and the couple had three children: two daughters and a son. Armstrong was known for his dedication to his profession and community, serving as clerk to the justices and achieving the rank of Major in the Royal Engineers during World War I. The family resided in an Edwardian villa named Mayfield in Cusop Dingle, Herefordshire.

In February 1921, Katharine Armstrong died suddenly, with her death initially attributed to natural causes. However, suspicions arose later that year when Armstrong's professional rival, Oswald Martin, fell ill after consuming tea and scones at Armstrong's home. Martin's father-in-law, a local chemist, had previously sold arsenic to Armstrong, who claimed it was for killing dandelions. The similarity between Martin's symptoms and those of Katharine led to the exhumation of her body, revealing high levels of arsenic. Armstrong was arrested on 31 December 1921 for the attempted murder of Martin and subsequently charged with his wife's murder. Despite maintaining his innocence, Armstrong was found guilty on 13 April 1922 and executed by hanging at Gloucester Prison on 31 May 1922.