Explain why it was easier for Pi to kill dorado as opposed to killing the flying fish?

From the text:

"You may be astonished that in such a short period of time I could go from weeping over the muffled killing of a flying fish to gleefully bludgeoning to death a dorado. I could explain it by arguing that profiting from a pitiful flying fish's navigational mistake made me shy and sorrowful, while the excitement of actively capturing a great dorado made me sanguinary and self-assured. But in point of fact the explanation lies elsewhere. It is simple and brutal: a person can get used to anything, even killing."

Pi cites that once you've killed, the killing becomes easier.