Since the arrests for Operation Cupid began, more than four hundred impostor machines that were known to be such had been arrested and executed, there were many more impostors in custody but data was still being verified. Penelope did not want to kill any innocent people by mistake.
But four hundred test subjects to execute were enough to know something indisputable, the machines had a physiology almost identical to the human one, however they came with certain improvements that gave them away quite a bit, and one of them was that despite being shot in the head, they could survive it, surely they would have some hidden regeneration mechanism.