Why does he consider killing the fish as sin?

Santiago believes killing the fish was a sin because he didn't kill it simply for food or to sell.... he killed it because he was prideful.

You did not kill the fish only to keep alive and to sell for food, he thought. 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?