The author depicts them as almost -wanting- to die, constantly. Sure, discipline can make you more corageous, but humans still have the basic primal instinct of survival, and it becomes pretty comic how willing they are to sacrifice themself for almost no reason at all.
Not discouraging other responses, but there was recently a post about the historical accuracy of Shogun that may be of interest to you. The impression I got from that thread is that the book does a pretty good job at depicting the Japanese culture of the time, but it's definitely not perfect.
Also, a note about Shogun: the main character looks at Japanese society as an outsider at the beginning, and it appears very cruel and barbaric. I don't want to give anything away, but as the book progresses the main character's opinions about Japanese society change quite a lot, and I assume, Clavell wants the reader's opinions to change as well.
hi! if you want to delve deeper into this, you may be interested in the book recommendation in this post
What is a good (non-fiction) companion book to James Clavell's "Shogun?"