Of tired, outstripped Five-Nines that dropped behind.
Gas! GAS! Quick, boys! – An ecstasy of fumbling,
Fitting the clumsy helmets just in time;
But someone still was yelling out and stumbling,
And flound'ring like a man in fire or lime . . .
Dim, through the misty panes and thick green light,
As under a green sea, I saw him drowning.
In all my dreams, before my helpless sight,
He plunges at me, guttering, choking, drowning.
If in some smothering dreams you too could pace
Behind the wagon that we flung him in,
And watch the white eyes writhing in his face,
His hanging face, like a devil'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
This is open to interpretation. The “hanging face” conveys misery, the jaws pulled down in terror and pain. The vivid “like a devil’s sick of sin” is a  hyperbolic , imaginative idea, the devil being the incarnation of sin.  It suggests also that not even the devil would be able to cope with the horror and inhumanity.