The part of this excellent and somewhat shocking poem you need to turn to is the last stanza, which comes after the gas attack, but describes what the other lucky soldiers did with the corpse of their fellow soldier and its physical appearance. Note the diction in the following lines which describe the corpse and the effect of the gas on the body:
And watch the white eyes writing in his face,
His hanging face, like adevil's sick of sin;
If you could hear, at every jolt, the blood
Come gargling from the froth-corrupted lungs,
Obscene as cancer, bitter as the cud
Of vile, incurable sores on innocent tongues...
The word choice of this physical description. The death of this soldier is anything but laudable and noble. On the contrary, his "white eyes" are said to "writhe," emphasising the pain and suffering he experienced. Likewise, his lungs are described as "froth-corrupted" as the poet paints an image by appealing not just to sight, but to sound as well. A series of similes nail the fact that this death was one characterised by immense pain and unimaginable suffering: his face is described as "a devil's sick of sin" and his poisoned lungs are described as being "obscene as cancer." This is no glorious and heroic death achieved against great odds as old poets would say. This is an ignomonious death against an unnamed and absent opponent that features pain, suffering and sorrow.