DECKARD: I don't know why he saved my life. Maybe in those last moments he loved life more than he ever had before. Not just his life, anybody's life, my life. All he'd wanted were the same answers the rest of us want. Where did I come from? Where am I going? How long have I got? All I could do was sit there and watch him die.
These next two quotes are both from the voiceover versions of Blade Runner. In this one, Deckard suggests that the reason Batty refused to kill him was because his own impending death made him treasure life more—his own and everyone else's.
DECKARD: I watched him die all night. It was a long, slow thing... and he fought it all the way. He never whimpered, and he never quit. He took all the time he had, as though he loved life very much. Every second of it... even the pain. Then, he was dead.
This voiceover quote doesn't really seem to match up with the movie itself. When Batty releases the dove, this seems to be the moment at which he dies, since the whole thing is chock full of symbolism. Nevertheless, this description shows Batty using his own terrible suffering to affirm life and give it a meaning.
ROY: We've got a lot in common.
SEBASTIAN: What do you mean?
ROY: Similar problems.
PRIS: Accelerated decrepitude.
Sebastian has a disease called "Methuselah Syndrome" which involves aging prematurely. So in this way, he's similar to the replicants, who have a shorter-than-normal lifespan—a mere four years.
TYRELL: What…? What seems to be the problem?
TYRELL: Death. Well, I'm afraid that's a little out of my jurisdiction, you...
ROY: I want more life, f***er.
Tyrell says death is out of his jurisdiction because he's subject to death himself. But he's only playing humble, since he personally had the audacity to create the replicants and determine what their lifespans would be, limiting them to a mere four years. So, he's perfectly willing to play God. Speaking of which, aren't Roy's sentiments things we can all sympathize with? A lot of people would probably like to ask God (or whoever) why they have to die, too.
TYRELL: The light that burns twice as bright burns half as long, and you have burned so very very brightly, Roy. Look at you. You're the prodigal son. You're quite a prize!
ROY: I've done questionable things.
TYRELL: Also extraordinary things. Revel in your time!
ROY: Nothing the god of biomechanics wouldn't let you in heaven for.
Roy reacts to Tyrell's consolations by killing him. Tyrell didn't create Roy to "revel in his time" and celebrate the brief moment given to him—he created him to serve as a slave. Tyrell calls Roy "the prodigal son" because the prodigal son returned to his father after running away and leading sinful life—and was rewarded for returning. Part of the irony here is that Tyrell won't reward Batty and can't extend his lifespan.
ROY: I've seen things you people wouldn't believe. Attack ships on fire off the shoulder of Orion. I've watched c-beams glitter in the dark near the Tannhäuser Gate. All those ... moments will be lost in time, like tears...in rain. Time to die.
Whereas Roy had previously raged against his mortality, he now accepts it with grace. He seems overcome by wonder at his own brief life, which makes him more forgiving.
GAFF: It's too bad she won't live. But then again, who does?
Gaff implies that the replicants' time-limited lives just present a more intense version of the human predicament. We all have an expiration date.