early in thre 21st century,the tyrell corporation advanced robot evolution into the nexus hase - a being virtually indentical to a human - known as a replicant.
the nexus 6 replicant were suoerior in strength and agility,and at least equal in intelligence, to the genetic engineers who created them.
replicants were used off-world as slave labor, in the hazardous exploration and colonization of other planets.
after a bloody mutiny by a nexus 6 combat team in an off-world colony, replicants were declared illegal on earth - under penalty of death.
special police squads - blade runner units - had orders to shoot to kill, upon detection,any trespassing replicant.
this was not called execution.
it was called retirement.