Mercerism -- Rick Deckard's Religion
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.
How Deckard Sees It
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. ... 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.