The theme of the novel that this excerpt best address is "Man and the natural world". In other words, how humans can see animals as a terrible monster for their own benefit. In this chapter, in particular, there are some rumors of Moby D. circulating among the whalemen, rumors that tend to put Moby D. as the most terrible monster of them all, capable of maiming and killing anyone, an evil creature (<em>malicious</em>) that will kill if he encounters anyone on his path. Also, Ahab talks about Moby D. as the worst and cruelest monster.
It's important to know that Moby D. is not an ordinary whale, it is very large and strange looking, his whiteness is very particular and the same goes to his jaws that are misshapen. Even if it has these tremendous characteristics its still a whale but in this book it's more like a mystical and fantasy character. It usually hard to find even if it has very clear characteristics.
In the excerpt "<em>monomaniac</em>" means an irrational concern with a single idea or object. We can see this idea reflected on Ahab's behavior as he wants to hunt and kill Moby D. because it would be a symbolic victory against the cruelty of this monster.
