Early on after heading out to sea, Santiago catches a tuna. He then strikes it on the head to stun and kill it swiftly. Hemmingway suggests that this violent act is done out of a sense of mercy for the doomed fish.
This little episode gets to the heart of Santiago's nature. He is a kind, thoughtful, and gentle person. Although his profession requires him to be violent towards the animals he catches, he is never needlessly cruel. He sees something of a kindred spirit in the creatures of the sea, as is evident by the way he often speaks to them as if they could understand him.
What we see in this example of hitting the tuna out of kindness is the dual nature of the old man. In fact, such a thing is a kind of oxymoron. An oxymoron refers to a contradictory figure of speech. Hitting and kindness are usually contradictions. It is also a contradiction to kill a creature whose life you respect. However, to Santiago, there is no contradiction here. He values the fish for their own lives, but also for the livelihood which they provide for him and his fellow fishermen. He is driven to kill fish, but he also pities and respects them.