when does the old man feel sorry for the marlin?

Santiago has a special relationship with the giant Marlin. He wants to catch it but he also reveres it. At times Santiago even calls for the Marlin to take his life. The old fisherman feels a kinship which transcends his relationship with other fishermen (except perhaps the boy). Santiago feels sorry for the Marlin as he feels sorry for himself. The Marlin represents Santiago in his prime, young beautiful and majestic. The Marlin is both foe and friend to the old man and, although victory over the fish is paramount, he feels sorry to kill it.

JUst to add. As the Marlin tires so does Santiago. All the old man's fatigue, sorrow and majesty is projected on to the fish.