Why did Eddie's father save Mickey instead of killing him or letting him drown for what he attempt with Eddie's mother?

Eddie's father saved Mickey because of his strong sense of loyalty.

"Mickey was coarse, but he was not evil. At that moment, he was lost, adrift, and what he did was an act of loneliness and desperation. He acted on impulse. A bad impulse. Your father acted on impulse, too, and while his first impulse was to kill, his final impulse was to keep a man alive." She crossed her hands over the end of her parasol.

"That was how he took ill, of course. He lay there on the beach for hours, soaking and exhausted, before he had the strength to struggle home. Your father was no longer a young man. He was already in his fifties."

"Fifty-six," Eddie said blankly.

"Fifty-six," the old woman repeated. "His body had been weakened, the ocean had left him vulnerable, pneumonia took hold of him, and in time, he died."

"Because of Mickey?" Eddie said.

"Because of loyalty," she said.