In The Old Man and the Sea, catching the marlin is usually discussed in terms of Santiago's success and failure. But there is a significant moment when Santiago does delve into notions about sin and pride. Although Hemingway never blatantly expresses how there is almost a spiritual connection between the old man and nature, Santiago does speak to the fish calling them "true brothers," as if they are all part of the same social and spiritual world.
Before the sharks come, Santiago starts to feel guilty and notes that "he meant me no harm" ("he" meaning the marlin). In fact, he explains the approach of the sharks not as an accident but as a kind of punishment. And although he was in financial troubles (and did need to fish in order to survive), he says:
You did not kill the fish only to keep alive and to sell for food. You killed him for pride and because you are a fisherman. You loved him when he was alive and you loved him after. If you love him, it is not a sin to kill him. Or is it more?
Santiago wrestles with this idea, later saying it was in self-defense (as a means of survival even if in the form of food and money at the market). Did he kill for survival or for pride? If it was for pride, it was a sin. The old man does claim that he "went too far." Does this mean that he physical sailed too far or that he went too far in terms of pridefully chasing the fish? Considering the old man feels defeated upon his return, it is logical to conclude that he feels it was a necessary sin. As he states earlier in the novel, after having killed the marlin, "Fishing kills me exactly as it keeps me alive." His guilt kills him but fishing allows him to live. I don't think Santiago concludes exactly whether or not it was a sin to kill the marlin, but I do think that he accepts that his guilt is a by-product of the world he lives in, thus giving him kind of a natural or providential justification. In other words, this is a part of nature or God's plan. He was born to be a fisherman. The fish was born to be a fish.