What simile is used to describe the gassed soldier's face in "Dulce et Decorum Est"?

Similes rather than metaphors predominate in the description of the gassed soldier; the central one, as  the previous post identifies, is His hanging face, like a devil's sick of sin as he lies in the wagon containing the dead and dying.

It is crucial not just to spot similes but to give explanations of how they work. This contains no less than 3  other poetic techniques! Sick of sin is alliterative and there is assonance allowing one to almost spit out the words. Most importantly there is also contradiction, oxymoron: how can a devil be sick of sin? The answer is that it is only his face which is like a devil's, made hideous by the effects of the gas. He himself is a man, sick of the evil of War.

It is vitally important to remember that a simile is a figure of speech that compares one object or character with another object that we would not normally link it with by using the words "like" or "as." It is not to be confused with a metaphor, that does the same thing, but asserts a direct comparison without the bridging of the words "like" or "as." Thus when we consider the graphic and repulsive description of the soldier's face we can see that the simile that Owen uses is as follows:

And watch the white eyes writhing in his face,
His hanging face, like a devil's sick of sin...

Thus the face of the gassed soldier is compared to "a devil's sick of sin" which effectively describes the almost unnatural and strong effect of the gas on the dead soldier's body. Note how it is effective in the strong way it describes the face of the soldier. The reference to the devil only serves to reinforce Owen's central point about how wrong war is.

Which metaphor in "Dulce et Decorum Est" describes the death of the gassed man?

Let us remember that a metaphor is a figure of speech that asserts a direct comparison between two objects that are not normally considered similar without the use of the words "like" or "as," as in a simile, which is the same except for the addition of these words. There are two kinds of metaphors in literature, direct metaphors, where the comparison is directly asserted, such as "the flower was a ray of light in a dark day," and indirect metaphors, where the comparison is inserted much more subtlely, such as "the teacher barked out his commands." Here you can see that the teacher is being compared to a dog through the indirect metaphor.

When we think of the dying soldier, we see that the metaphor used to describe his manner of dying relates to the green panels of the gas mask through which the poet sees him:

Dim, through the misty panes and thick green light,
As under a green sea, I saw him drowning.

Note how there is an indirect metaphor in the word "drowning," which compares the way the soldier dies from the gas to a man drowning in the sea.