... what a terrible way to find out someone was KIA ...
Yes, but only if
this is how they found out.
As you suggest, looking at this envelope and slowly 'figuring it out' would, indeed, have been the worst way to learn the worst.
But WW2-era notifications were largely by telegram ... cold, yes, but at least you knew it was some sort of bad news before you read it.
So, rest assured, this family knew their loved one was gone long before this letter was returned ... albeit re-opening their wound, and adding a new layer of loss ('he never got to read this').
On a related note, what does the cartoonist mean by "Somme Post Office"?https://stamporama.com/discboard/di...=20&id=12480
... a thread about the mail of soldiers KIA, which included the postcard (above) and this statement:
From February 1915, all returned mail endorsed "Missing" or "Killed" was first sent to the Home Depot in the U.K. There, it was sorted into bundles and sent to the appropriate Record Office; or in the case of officers, to the War Office. It was held until relatives had been informed of their loss.
/s/ ikeyPikey (whose family received one or two of his father's letters home several days after he passed, and remembers that moment, 50-odd years on)