what are the old man's arguments with himslef about killikng the big fish?

The old man finds the great marlin majestic and noble. He thinks that the fish deserves the respect not to be caught. As he fights the fish Santiago calls him his brother and, at one point, considers sacrificing himself. Still, Santiago can't bear the thought of a lesser man catching this great fish. Out of respect for the fish, he fights on.

The fish is also his one chance to prove himself still a great man.