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.