Re: Deckard
I always thought the book was heavily implying that EVERYONE was a replicant aside from the chicken heads. Like, the radiation level was so high that any natural-born human would have severe birth defects, including extremely stunted mental development. The whole society was just replicants worshiping mechanical animals and continuing to follow their programming - including the blade runners, who were programmed to hunt down and kill other replicants when the humans were no longer capable of doing so themselves.
Roy defeated Deckard easily because Roy was a later model.