The first recorded killing of a human by a Cylon was ten years before the Rebellion, in a mid-sized city on Virgon. Local security forces were called out one morning when neighbors reported hearing a loud, ongoing disturbance from the home of a reclusive electrical engineer. They arrived on the scene to find the man's head half-crushed by the Cylon model he'd purchased for household chores (and, one particularly prurient neighbor told local news sources, "companionship.") The Cylon was locked in a destructive mode, flailing its heavy metal limbs at anything in its path: walls, furniture, objets d'art, officers of the peace.
Further investigation revealed that the victim had evidently been tampering with the Cylon's circuitry, which resulted in crossed wires and systemwide malfunction. It was labeled "death by misadventure" and the case was closed.
The second recorded killing of a human by a Cylon was two years later on Sagittaron, when a joint human-Cylon work crew at a tool manufacturing plant was just about to go off-shift. The crew's foreperson fell to her death from a catwalk four stories above the shop floor. The Cylon worker on the crew had been walking abreast of the victim on the narrow footbridge. A low rail guarded the drop-off, but it was surmised that when a load of raw materials suddenly crashed to the floor, it caused the Cylon to tilt to the side, falling against the victim and sending her through the guard rail and over the edge.
The Cylon's safety programming and gyroscopic balance was found to be lacking, and the developer fined. Though the incident was officially considered an industrial accident, worker's-rights groups on Sagittaron rioted until the Cylon was melted down for scrap metal. The victim's family settled out of court with the Cylon manufacturer and the employer for an undisclosed sum. Months later, the victim's daughter appear in an interview on The Colonies Today, asserting that her mother had always hated Cylons and didn't want them on her crew. And look, the daughter said, she was right.
By that time, the incident the tabloids had dubbed "the Virgon sex-robot scandal" was all but forgotten.
The third killing of a human by a Cylon was never recorded or reported.
No more than half a dozen people in a government R&D lab on Caprica ever knew about it. At the time of Caprica's destruction by the Cylons, the victim was still listed as "missing" on official law enforcement records.
The third reported killing seemed a little different.
In a remote region of Gemanon, a self-styled oracle had constructed a temple to Hera and sat to await pilgrimage from homemakers and mothers across the colonies. Despite the well-known faith of Gemanon, no one came.
Until one day, five years before the Rebellion, a gang of thieves and other unsavory sorts came up from the south to do gods knew what in the unforgiving badlands surrounding her temple. When they stumbled on it, they desecrated the temple, raped the oracle and sacked her belongings. She collapsed to her knees in prayer to the gods, but somehow she still ended up with a broken neck.
The oracle's one companion all the long empty years had been one of the first Cylon models equipped with artificial intelligence; her steward, devotee and friend. The sole surviving member of that gang told of how the Cylon hunted them down, one by one in the vast temple, and executed the judgment of the gods.
The people of Gemanon cared more for the oracle in death than they ever had in life, and they praised all Cylons' fidelity and justice, naming them the hands of the gods.
They introduced a movement in the Quorum to add weapons-equipped Cylons to security forces. An army of cybernetic soldiers, they called it. A superhuman protector of the people wherever human officers could not be present.
This was not humanity's first mistake regarding the Cylons, nor was it even the greatest. It was, perhaps, the most pivotal.