I think that there can be many different points to illuminate the importance of Billy Boy Watkins's death in Going After Cacciato. One of the most important would be that the soldiers who fight in wars are not monolithic and devoid of human expression. Billy Boy dies out of fright, the victim of a heart attack. Contrary to the single dimensional aspect of the soldier who is stoic in the face of certain death, Billy Boy comes to represent the human element of those who fight in war. No amount of preparation enables the individual to fully understand what it is that they experience in war. Billy Boy was not equipped to address what he encountered. As a result, he dies. This reflects the importance of his death because it speaks to what it means in the soldier as human being.
Another reason why his death can be seen as important is because of the reaction it causes amongst the other soldiers. The other soldiers use Billy Boy's death not as a warning of what might await them as much as a force of division between themselves and the harsh condition of war. Billy Boy's death becomes immortalized in a song, something that the soldiers sing in almost a joking manner. Billy Boy's death is a means by which the soldiers are able to construct a barrier between themselves and the harsh reality of the world around them. This is where one can see how Billy Boy's death is important. While it reflects the condition of war that all soldiers face, those who are in harm's way cannot accept this reality in order to fulfill their mission and survive it. Rather, they seek to find ways to divide themselves from such a condition and the death of Billy Boy is one such way in which that division is evident.
I would suggest that the final way in which that Billy Boy's death is important is that it is indicative of one of O'Brien's themes. Billy Boy dies of fear, of a heart attack that is brought on by fear. O'Brien stresses that Billy Boy's lack of control over his own fear helped to bring about his death. One of the critical themes in the novel is the examination of the components of courage in war situations. The ability to generate courage and a sense of control in the most painful of times can have a determinant impact on success and failure, life and death. Billy Boy lacks this control and courage and pays the ultimate price for his inability to demonstrate the necessary elements to confront that which awaits him. In his death, one is reminded of both the painful condition of being human and yet the almost transcendent demands that soldiers must embrace when placed in harm's way. This thematic emphasis becomes another reason why Billy Boy's death is important to the development of O'Brien's work.