In "Dulce et Decorum Est," how does the poet impress upon us the suffering of the gassed man?

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:

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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.