I love questions like this because there's actually loads to break down within it.

To begin with I'm going to suggest a few pieces of general reading around the portrayal of the First World War culturally and through various media. This will be a bit Anglocentric but then I get the feeling that the question is primarily interested in British understandings of the war.

So the following books might be useful to you:

'The Great War: Myth and Memory' by Dan Todman

‘Mud, Blood, and Poppycock' by Gordon Corrigan

'The Last Great War' by Adrian Gregory

'The First World War in Computer Games' by me (sorry).

However, before we can really even get into the portrayal of the war in media, there's some aspects that we need to deal with first.

The War

Primary with this is the idea that the war was welcomed in 1914 (something worthy of being lionised). Part of that is true but, at the same time, the idea of a huge popular 'rush to the colours' isn't entirely true. Most of the major protagonists often saw the war as something being inflicted upon them and that to lose it would be apocalyptic to their society. As a result it becomes a righteous cause to save France/Germany/Britain/etc from its enemies but it was an enthusiasm in the face of annihilation.

That being said, in order to get people to fully buy into the war (a concept that John Horne refers to as 'self mobilisation') socieities had to go through various procedures which often involved putting on hold their own existing political differences and rallying around the national cause. I wrote a bit about that in regards to France, [here] (https://www.reddit.com/r/AskHistorians/comments/edt2fi/how_did_the_french_left_respond_to_the_outbreak/)

Now part of producing that national support and consensus was through the use of patriotic poetry and media espousing the necessity of the war and the glory of serving in it. But given that Britain was the only country that didn't have compulsory military service, a lot of these attempts were aimed at getting men into the army.

So if you look at poetry (for example) from 1914 and 1915 a fair portion of it will be about the potential for heroism but also the benefits to Britain and civilisation for standing up against German aggression.

But, as the war goes on, and any initial hopes for an early victory dissipate the portrayal of the war does begin to change, but it's not quite in the manner you predict here:

but as it dragged out for years and years people largely became sick of it and thought of it as pointless slaughter.

1916 is a particularly important year for this, especially for Britain and France. For the British the huge volunteer armies that were raised in 1914 & 1915 are deployed to the Western Front for the first time and fight en masse at the Somme. For the French they fight almost all year in a huge attritional battle at Verdun which sees almost the enitre French army cycle through the area and becomes a huge national effort.

Both of these battles see huge casualties and neither, at least immediately, seem to end the war. The opening of the Battle of the Somme is captured by documentary makers and screened in cinemas back in Britain almost immediately afterwards. The film (famously) includes staged scenes of men going over the top and seemingly dying but it also repeatedly features dead bodies of British and German soldiers lying around all over the place and is edited in a manner that, at times, makes it look like living soldiers march to the trenches, go over the top, and die.

In the first 6 weeks of its release 20 million tickets were sold for it. Britain at the time had a population of around 48 million, so a significant proportion of the population see it. But it doesn't cause support in the war to collapse and people don't see the conflict as a pointless slaughter after seeing it. Nor do those French families who lost men at Verdun.

What this period does is harden the resolve towards the war rather than weaken it. The only way the war becomes 'pointless' now is if the British or the French lose. Then all these sacrifices were for nothing.

Now it's true that as the war continues through 1917 and into 1918 you see the beginnings of unrest at home and in the various armies. There are strike actions on the British homefront, and the French army mutiny in 1917 but I would argue these acts are not a rejection of the war, more a rejection of the difficult conditions faced in waging it. But morale in the allied armies never collapses to a point where the British or the French think the conflict is pointless. Right up until the end they believe not only is it a war that can be won but it is one worth winning.

Obviously during this time-period you do also see poetry and prose being created which espouse the horrors being faced in the trenches by these soldiers, but a lot of this writing does not gain traction until the interwar years...