Prometheus: a natural sequel of 1982 Blade Runner

Deckard: [narrating] 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.

...at least, for Ridley Scott. Deckard seems like the voice of the Legendary director in the 1982 movie "Blade Runner" and these words are kind of repeated by Doctor Peter Weyland in the movie "Prometheus": 

Peter Weyland:

Weyland Corporation: Building Better Worlds. Hello, friends. My name is Peter Weyland. I am your employer. I am recording this 22 June 2091 - and if you are watching it, you have reached your destination, and I am long dead. May I rest in peace. There is a man sitting with you today - his name is David - he is the closest thing to a son I will ever have. Unfortunately he is not human. He will never grow old and he will never die - and yet he is unable to appreciate these remarkable gifts, for that would require the one thing that David will never have: a soul. I have spent my entire lifetime contemplating the questions "Where do we come from - what is our purpose - what happens when we die?" And I have finally found two people who have convinced me they're on the verge of answering them: Doctors Holloway and Shaw - if you would please stand. As far as you're concerned, they're both in charge.The Titan Prometheus wanted to give mankind equal footing with the gods - for that he was cast from Olympus. Well, my friends, the time has finally come for his return.

Is David the "natural" evolution of Roy Batty?

What are the differences between the disappointments of Roy Batty and David with their own creators, Dr. Eldon Tyrell and Dr. Peter Weyland?

Roy Batty wants That his creator extends his four-year lifespan and David doesn't have a soul, according to Peter Weyland. I disagree of Weyland. The defect of David wasn't real, it was a perfect creation of a synthetic human being with a soul and that scared Peter Weyland, because his creation was beyond his comprehension.