I've always wondered why Dr. Tyrell didn't simply lead Roy Batty to believe that there was some experimental method they had been working on to extend a Replicant's four-year lifespan, or even a proven secret method. They would no doubt had to have put Batty and Pris through some sort of procedure that might anesthitize them. Once under all they would have to do is "retire" the two of them humanely. I mean, knowing his own life was at risk I wonder why he didn't proceed along those lines in an attempt to save himself. Not that the scene isn't powerfully written and performed as is. Just a thought.