In the story, Juvencio state that “he had to kill Don Lupe.” What does this line reveal about how he views his deed? What do his further thoughts in lines 41–101 make clear to readers?

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.

Juvencio says that he had to kill Don Lupe because “he had refused to let him pasture his animals," in spite of him being Juvencio’s friend and the owner of Puerta de Piedra. There had been a great famine and Juvencio hadn’t really understood how Don Lupe could claim to be his friend, yet refuse to allow him to graze his animals on his pastures, even as the animals died in their numbers. Driven to the wall, Juvencio had broken through the fence of Puerta de Piedra to allow his poor animals to graze his friend’s lands in the dead of the night. After repeatedly breaking into his friend’s farms, his friend retaliated by killing one of his animals. It is at this point that he killed his friend.
When Juvencio says that “he had to kill Don Lupe," he seems to believe that his deeds were justified and that killing Don Lupe was the only way he could resolve the conflict between him and his friend at the time.
In the lines immediately following this statement, he further explains why his actions were justified. The reader clearly sees that Juvencio placed a lot of value on his animals. He valued them more than he valued the life of his friend. Thus, to him, his friend was to blame for the murder. Even as he faces his own death in the hands of the colonel, he does not regret his deeds those many years ago.