Sometimes our traumatic pasts serve as our own caves; we stay confined in them and cannibalize on those pasts. To the point we can't even trust our closest relationships anymore.
So yes, the monsters were once human.
The Descent is littered in guilt and sadness, which is why it still works for me (on top of Neil Marshall's claustrophobic direction). But man, I do wish the studios kept the original UK ending; those specific final minutes are so crucial to the film's emotional core, and necessary.