Because this is such a negative poem, there are all sorts of words in it that have negative connotations. Together, the words (as well as the overall action of the poem) make us feel very negative emotions towards the war and towards the havoc that it wreaks on the young men who are fighting in it.
If you look at the first stanza, there are all sorts of negative words. We see the words "beggars," "hags," "sludge," "haunting,"blood-shod," "lame," "blind."
The second stanza has fewer of these words -- I see "drowning," "choking," and "guttering."
But then you look at the third stanza and it is full of negative words. For example, "obscene," "smothering," "writhing," "vile," and "incurable."
When you put all of these together, you have a very horrible feeling towards this war.