Nelson DeMille can be quite dark. Spencerville, The General's Daughter, and even Plum Island come to mind. The more books he writes, the darker he becomes, it seems.
I have been a fan since Charm School. I worked my way through Gold Coast, Cathedral, Rivers of Babylon and even Mayday, one of his earlier, co-authored, often overlooked books. Mayday shows the signs of his greatness.
DeMille works best for me when he maximizes the humor (often irony) and tension while minimizing graphic viloence or the dark side of human nature. Often his villains are driven but not inherently evil. His heroes are always flawed and usually funny in the worst situations.
Word of Honor balances all of these features and the result is one of DeMille's best, if not THE best. Vietnam has been cathartic for many Americans. Unlike World War II, where good and evil seem clear (and the basis for more heroic fiction than any other event in the world), Vietnam is nuances and conflicted lives. DeMille uses these to fine tune his humor and his hero.
This one is well worth checking out.