I encourage you to look at chapter 23, "Monster," in the book, for the very first descriptions of the Bird (or Corporal Watanabe), as they seem to sum him up the most directly. In his first interactions with Zamperini, he strikes the POW directly on the head, twice, for not looking him in the eye. The sentence following is perfect:
This man, thought Tinker, is a psychopath (232).
More background information is provided for him in the pages that follow, including a brief description of his childhood and entrance into the service. Here, it is first established how deeply patriotic Watanabe was, and how his generation was raised in a way that seemed to desensitize them to violence.
Throughout the imprisonment, the Bird only grows more and more sadistic in his...
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