The conflict between the blade runner Deckard and the off world replicants is the central force of Ridley Scott's Blade Runner. This conflict not only provides the means of narrative movement, but the philosophical and symbolical stimulation that engages one's mental and visual appetite.
I've seen things you people wouldn't believe...Attack ships on fire off the shoulders of Orion... I watched c-beams glitter in the dark near the Tannhauser Gate. All those moments will be lost in time...like tears in rain. - Roy Batty.
The argument that makes the above conflict interesting is the possibility that Deckard was that which he had to kill: a replicant. This argument has been posited by many critics of the film, and has further intensified my understanding of the film. In this paper I will not only use the great body of criticism to defend this argument, but present a theory of my own: that Deckard's nemesis, Roy Batty, knew that Deckard was a replicant.
The likelihood of Deckard being a Replicant has always been an overt possibility in the film. Rachael, a replicant whom Deckard administered the Voight-Kampff test to, asks him if he himself has ever taken the test to find out if he is truly human. The question goes unanswered, but further elements of the film make the silent answer all the more apparent; such as those noted by Philip Strick  :
Four replicants have arrived in the city - Pris, Zhora, Leon, and their leader Roy Batty (Rutger Hauer) - and Deckard has to deal with them. Minutes later, Bryant describes how six replicants ("three male, three female") hi-jacked a space shuttle back to Earth, and one of these has since been killed during a break-in at the Tyrell Corporation headquarters. Which leaves five.
Further, the great importance of visual and memory design within the film leads us to find that Deckard is not much different from those whom he hunts.  ("How does it feel to live in fear?") The replicants rely on photographs and implanted memories to bolster their nascent and fragile emotions. After Deckard tells Rachael that her photos and memories are merely copies of those that belong to Tyrell's niece, he falls asleep amidst his own childhood photographs.
Perhaps the most important aspect of the recently released director's cut is the footage of Deckard's dream. He dreams of a unicorn. This is directly referenced at the ending in which another blade runner, Gaff, leaves an origami Unicorn outside Deckard's door to signify that he is allowing Deckard to escape with Rachael. By this inclusion, Scott lends weight to the "Deckard as a replicant" concept by implying that another blade runner knew Deckard's dreams. Nor was Scott above playing with words as seen by the fact that Deckard "retires" (kills) Replicants, after he himself is brought back from retirement.
The above interpretation of Deckard has led to much discussion concerning the nature of Deckard's relations with the replicants and especially Roy Batty.
First, I shall consider Deckard's relationship with the other replicants. In a grotesque way it is one of melding, marriage, and death. Deckard finds himself in love with Rachael, a replicant he should kill. Later, he seeks out Zhora and approaches her as she showers in order to protect her from "lewd and unsavory persons" and "dirty holes". This sexual context is a pretext to determine whether she is a replicant. She flees, and he chases and kills her in the slow scene of shattering glass and blood. When Leon attempts to avenge Zhora's death, Rachael kills Leon, further indebting Deckard to Rachael.
The scene in which Deckard kills Pris is important as well, as seen by David Dress's interpretation:
Later, Pris, who knows Deckard is coming, puts on a long veil and hides among the dolls in J. F. Sebastion's apartment. As Deckard sees her and starts to remove the veil, like a bridegroom approaching his shy bride, she attacks him. She performs two lightning-like flips, leaps high in the air and lands on Deckard's shoulders, crushing him between her thighs. 
Hence, with Deckard's relations to the other replicants seen as one of marriage or the incorporation of that which he kills, we can further consider his relation with Roy.
Roy has often been interpreted as a Christ/Anti-Christ figure, and there is much evidence to support such an interpretation: Roy visits his father Tyrell atop Mt. Olympus to confess to his creator, he pushes a nail through his own hand in order to feel pain and "life", and when he dies, a white dove ascends from his still body.
Considering Deckard in the same light, we see that Deckard is possibly not "human" as well, but a replicant. One who has risen from the dead (retirement) with definite human qualities, so as to be the Son of Man and Replicant. This union is accomplished knowingly by Roy, forging Deckard through the fires of a harrowing battle. Through this, Roy tries to communicate his life experiences, and the importance of life before his own flame extinguishes.
Of course, the question of how Roy knew about Deckard need be asked. Aside from the possibility that Deckard could be one of the "missing" two replicants, who could have known Roy before Deckard's mind was reprogrammed by Tyrell Corp., a previous scene acts as the precedent for a more likely theory. Off screen Deckard is able to read Rachael's detailed files on her inception date, and the source of her memories. If Gaff had access to such files on Deckard (as seen by the unicorn) it is probable that Roy would have seen similar information on Deckard as he searched for information on himself after he killed Tyrell. 
Hence, the riveting ending is not a simple chase by which a replicant chases a human, but a melding. A further transference (marriage/sex) between Deckard and Roy as seen by Roy's playful exhortations for Deckard to live:
Come on, Deckard, show me what you're made of... Proud of yourself, little man? My turn.... I'm going to give you a few seconds before I come. I'm coming... Four, five, try to stay alive. Come on, get it up!... Unless you're alive you can't play. And if you don't play... [you're dead]. Six, seven, go to hell or go to heaven.
Considering this, it is arguable that the conflict I mentioned, is a dialectical process by which Deckard and the other replicants merge super-human abilities with human frailty and life. In the final scene, Roy further prepares the way for Deckard, by transforming into a dove, which parts the black fog that has permeated the film. This final action allows Deckard and Rachael (Adam and Eve) to part from the clouds as well, to an unpolluted garden; taking the memory of Roy's tears, so as not to be lost in the rain.
Written by Joseph M. Reagle Jr. , 1997
Strick, Phillip. "Blade Runner Telling The Difference: Does the director's cut show that Deckard is a replicant?" Sight and Sound. (12/2/92) p. 8. ↩
Deutelbaum, Marshall. "Memory/Visual Design: The Remembered Sights of Blade Runner" Literature/Film Quarterly (17.1) 1989 : 66 - 72. ↩
Dresser, David. "Blade Runner. Science Fiction & Transcendence" Literature/Film Quarterly (18.1) 1990 : 172 - 178. ↩
Discussions between Madeleine Jarolimek and Joseph Reagle Jr. ↩