In "Where Have You Gone, Charming Billy?" by Tim O'Brien, Paul Berlin laughs uncontrollably about Billy Boy Watkins' death because he is suffering from combat and operational stress reactions.
Throughout the story, Paul Berlin experiences intense fear, anxiety, and guilt, which are all symptoms of combat and operational stress reactions. When Billy Boy dies, Paul is overwhelmed by a mix of emotions, including grief, fear, and a sense of disbelief. His laughter is not a reflection of his joy or relief at Billy Boy's death, but rather a symptom of his psychological distress.
O'Brien portrays Paul Berlin's laughter as a coping mechanism that helps him deal with the trauma and horror of war. By laughing, Paul is able to temporarily dissociate from the reality of Billy Boy's death and the violence that surrounds him. However, this coping mechanism is not sustainable, and Paul eventually confronts the full extent of his trauma and its impact on his mental health.