They were a threat. How could they not be?
It was not, strictly speaking, their fault; but they were there, and they could not be ignored.
Simple creatures of appetite and action; dangerous to a degree, but controllable.
They could be kept.
(Picture the camps; orderly rows of huts and outbuildings; rationality and cost-efficiency overlaid on the bedrock of horrifying abomination.)
(Because you have a by-product; and by-products all too often go astray.)
Someone joined the dots; and made a monstrosity.
The day they discovered that zombies taste like chicken.
That was … the day that conscience died.