There’s been at least 4 or 5 years since I saw the movie so it may be there and I don’t remember. I don’t understand the whole purpose of hunting Replicants.
For the most part they seem to be hiding and living normal lives, not wanting to be found.
They seem to want to be left alone anyway.
They have an expiry date anyway so even if you don’t hunt them they’ll eventually die.
So what’s the point of the blade runners? Is it explained why replicants need to be hunt?
Because they're basically amoral psychopaths, hence their incapacity to fool the VK test. Notably, many of them have killed humans to get to Earth and have killed on more than one occasion once they've arrived. If it was in the benefit of a replicant to kill you, it would do so without any compunction, in the same way that you might squash a bug that was annoying you, and with no more empathetic feeling.
In the original source novel, Rick describes them as "solitary predators" living among the human population.
Evidently the humanoid robot constituted a solitary predator.
Rick liked to think of them that way; it made his job palatable. In retiring—i.e., killing—an andy, he did not violate the rule of life laid down by Mercer. You shall kill only the killers, Mercer had told them the year empathy boxes first appeared on Earth. And in Mercerism, as it evolved into a full theology, the concept of The Killers had grown insidiously. In Mercerism, an absolute evil plucked at the threadbare cloak of the tottering, ascending old man, but it was never clear who or what this evil presence was. A Mercerite sensed evil without understanding it. Put another way, a Mercerite was free to locate the nebulous presence of The Killers wherever he saw fit. For Rick Deckard an escaped humanoid robot, which had killed its master, which had been equipped with an intelligence greater than that of many human beings, which had no regard for animals, which possessed no ability to feel empathic joy for another life form’s success or grief at its defeat—that, for him, epitomized The Killers.
This question is answered fairly simply in the film. Immediately after the opening credits, we read the following introductory text:
Early in the 21st Century, THE TYRELL CORPORATION advanced robot evolution into the NEXUS phase - a being virtually identical to a human - known as a Replicant.
The NEXUS 6 Replicants were superior 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.
We don't have any more information on the exact reasons Replicants were declared illegal on Earth. We might guess that it was either a retaliatory, collective punishment, or perhaps a genuine safety precaution.
So, these orders and the law they were based on are the reason for the Blade Runners' actions, in general.
Later in the film, when asked about his feelings towards the Tyrell Corporation, Deckard explains his own, unemotional take on the situation:
Replicants are like any other machine. They're either a benefit or a hazard. If they're a benefit, it's not my problem.
Apparently, if we're to take him at his word, Deckard feels that the Replicants he has retired were posing a hazard, rather than providing a benefit.
