After Paul fatally wounds the French soldier in the shell hole, he begins to realize that the soldier is just a man like him. Throughout the novel, Paul has been saying that the war has robbed him and the other soldiers of their youth and their humanity, so fear and the urge to protect himself take over Paul when the soldier drops into the hole. Paul tells the soldier that if he would have only recognized that they were both human then Paul would not have had to kill him. He makes promises to live the life of the French soldier as a means of retribution for his actions--Paul feels that since he has robbed the soldier of his life, it is now his duty to carry out that life as the soldier would have lived it. He later decides not to keep these promises because he cannot face the man's family. Paul's extreme guilt and confusion are revealed in this section of the novel.
This scene highlights the overall theme of the novel that is captured in the epigraph at the beginning of the novel:
This book is to be neither an accusation nor a confession, and least of all an adventure, for death is not an adventure to those who stand face to face with it. It will try simply to tell of a generation of men who, even though they may have escaped shells, were destroyed by the war.
Paul now knows what it is to kill a man face-to-face, and the experience destroys any sense of youthful innocence that remained inside him.