When looking at the point-of-view in Guy du Maupassant's "The Grave", one could very easily become confused. Typically, the use of the pronoun "I" would lead one to believe that the point-of-view is first person. Here, it is not the case.
If one were to look at the opening of the story, they would realize that the story is narrated in third person given the use of the pronoun "he" and the use of the proper names "Vincent" and "Courbataille".
The appearance of the pronoun "I" is used so that the narrator can remit to the reader what Courtbataille said while in court.
Therefore, while "I" is used throughout the story (typically notating a first person narration), here, it is used to quote what a character, Courbataille, says when defending himself.