Answer: a more negative connotation than the word “unpleasant.”
The word "unpleasant" is used to describe something that does not make us feel good. It refers to something disagreeable that makes us feel uncomfortable. The author, however, is describing images of people suffering and dying. He considers these to be much worse than simply uncomfortable. The word "monstrous" conveys this feeling better, because it has a more negative connotation.
