The statement that best explains how details in the passage develop the central idea that Boxer is kindhearted and does not want to harm even an enemy is:
The details show that Boxer is upset at the thought of killing the boy.
This is supported by Boxer’s sorrowful reaction to the boy lying in the mud and his emotional response, including having tears in his eyes and expressing his intent to not take life. These details illustrate Boxer's kindhearted nature and his deep remorse over the incident, highlighting his reluctance to harm others, even those who are considered enemies.