Imagery is most clearly used to create a negative mood in point B. Beneath the surface of the putrid swamp cane a gurgling that suggested the ghosts of those who died here were drowning still.
The creation of mood starts with diction (the choosing of words from the writer). Emotional implication of words is the most important factor to create a certain atmosphere. In this case, the negative connotation of words builds up a particular imagery that creates a negative perception for the reader, which eventually leads to a negative mood.