Nexus-6 is a type of andriod replicant
found in "Do Androids Dream Of Electric Sheep?
" and "Blade Runner
" who possess superior strength, agility, and intelligence
compared to the genetic
engineers who created them. They were identical to humans
These replicants were used in the off-world colonies as slave labor, mostly in hazardous exploration and colonization of other planets. They were declared illegal on Earth - under penalty of death.
To determine if a being is replicant or human, the law enforcement used the Voigt-Kampff empathy test. The problem was that Nexus-6 models were so advanced that the test had a difficult time delineating the emotional response. Rachel Rosen was the first Nexus-6 to be correctly identified by Rick Deckard as a replicant.
Those identified as replicants were done for.
This was not called execution.
It was called retirement.