What part of a story offers insight into human nature

Answer: Theme.

Explanation:

The theme of the story is a part of the story that conveys the central idea of it. Oftentimes, through the theme of the story, the author criticizes/praises certain aspects of human nature. The theme could be, for instance, maturity, corruption, friendship, etc. The theme tells us something about the characters and their actions, and explores certain phenomena of human nature which are usually present in real life.