In the Victorian and Georgian periods, the killing of a fox other than by hunting was considered by many to be tantamount to a crime, de facto, if not de jure (and referred to semi-seriously as 'vulpicide'). Shooting a fox mid-hunt would, in particular, deprive the hunt of its raison d'être and ruin a perfectly good afternoon for a hundred or more people.
Note that foxes in this period were almost taken to extinction by trapping, poisoning and shooting whereas the huntsmen would often intentionally breed foxes and have them left alone, better to allow the hunt to take place. Without the hunt, the wild fox would be extinct at this point in time.
An 1862 report of one such incident appeared under the heading ‘Atrocity In The Hunting Field’ as a correspondent who signed himself ‘Bullfinch’ related:
The Duke of Rutland’s hounds met on Saturday, the 27,h inst. at Croxton Park and drew Coston Cover where they found a fox starring him towards Buckminster ... but a hard fate awaited him as an individual (it would be libel on the name to call him a man) shot him within two fields of the cover just as the hounds had warmed to their work.
The noble master rode up to the infidel who had hidden the murdered fox in a barn and demanded that poor Reynard should at once be given up, but the assassin, gun in hand, placed his back to the door and swore he would shoot the first man attempting to enter. However, his cowardly threat was held at its worth and the Duke, dismounting, managed to wrest the gun from him...
I was told the fellow’s name is Marshall occupying a farm in the neighbourhood of Buckminster. Such an outrage cannot but obtain for him an unenviable notoriety and I would suggest that every honest man should forthwith ‘send him to Coventry’ ... and thus, by drawing the attention of the sporting world to the outrage, banish the creature from civilised society.
The Story of Your Life: A History of the Sporting Life Newspaper (1859-1998)
and
As foxhunters took to breeding foxes, a clear contradiction in the
rationale of the sport emerged. The hunt was ostensibly there to serve
the needs of the farmers: it existed to kill the vermin that plagued
their farms. But the foxhunters also insisted that they alone were
entitled to kill foxes and woe betide any farmer who dared to take the
destruction of foxes into his own hands. In the 1820s, a new word
entered the English language. In that decade, the Sporting Magazine
began referring to ‘vulpicides’ - by which it meant people who killed
foxes with utilitarian, rather than sporting, intentions.91 In the
absence of any legal sanction prohibiting the killing of foxes, a
social convention emerged, and as the foxhunters assiduously imported,
bred and reared a new population of foxes, so did they successfully
implant the notion that vulpicide was a despicable act. The American
novelist Harriet Beecher Stowe, visiting England in the 1850s,
observed, ‘It seems that killing a fox except in the way of hunting is
deemed among hunters an unpardonable offence.’ Her observation was
entirely correct.
Blood Sport: Hunting in Britain Since 1066