When he states that "he had to kill Don Lupe," Juvencio reveals that he still thinks of the deed as morally justifiable and necessary—so necessary that he had no other choice.
In lines 41–101, Juvencio says that the murder happened "thirty-five years ago." He thought that it had been "buried" or forgotten because it was "so long ago," the implication being that he thinks any responsibility he might bear for the murder has been "buried" beneath the "thirty-five years" that have passed.
Juvencio explains that the murder of Don Lupe was "Not for nothing," but because Don Lupe "refused to let him pasture his animals" on the land (the Puerta de Piedra) that he, Don Lupe, owned. Consequently, Juvencio's animals began "dying off one by one." Juvencio responded by cutting a hole in the fence surrounding Don Lupe's land so that his animals could graze there. In response, and by way of a warning, Don Lupe killed one of Juvencio's animals. In lines 41–101, it becomes clear that Juvencio considers Don Lupe's actions to have been grossly unfair, vindictive, and deserving of murder. As far as Juvencio is concerned, he "had to kill Don Lupe" from a moral perspective and also for practical reasons, so that no more of his animals would die.
Juvencio also says, revealingly, that all of Don Lupe's children were taken away to live "far off with some relatives," which meant that "there was nothing to fear from them." This latter quotation is revealing, because Juvencio doesn't express any sympathy for Don Lupe's children but only relief that they were taken far away so as not to be a threat to him. This suggests that Juvencio is a particularly selfish, remorseless character, especially given the brutal nature of the murder revealed towards the end of the story.
Juvencio also says in this passage that he has repeatedly paid off the judges (with, for example, "ten cows") and "the rest of the people" who have come looking for him, implying that he thinks he has more than paid his dues for any responsibility or guilt he might have, or rather might have had, for the murder.