Symbolism, Imagery, Allegory
Anger is another motif in The Demon's Lexicon, and it's one of the things that's used to emphasize the differences between Nick and other characters in the book.
Although Nick often comes across as unemotional, anger is an emotion he feels intensely. So intensely that it worries him, in fact. For instance, after Alan gets marked by a demon, Nick is enraged at Mae and Jamie because he thinks it's their fault. It's here that we get the line, "He wished sometimes that he could feel angry without feeling the urge to kill, but he never had" (3.87). Gulp.
Later, as he and Alan are driving home from the Goblin Market, Nick again feels immense anger and wonders "if other people got as angry as he did" (6.50). Throughout the story, we see Nick battling to keep his anger under control. There are times when he wants to hit his brother (2.175) or fly at people's throats (14.56), and while we've all probably had moments like that, the frequency with which Nick reaches such a high level of anger indicates he might have a bit of an issue.
That said, Nick does manage to keep his anger in check most of the time, so by analyzing this motif, we can probably draw more than one conclusion about what it means. Might we think of anger as an inner demon?