their gene pool is damaged and they breed a lot of defective spawn.

it's nature's way of discouraging in-breeding.

They insist on keeping it in the family bloodlines.

The often quoted and mocked tales of them being reptilians actually may have some basis in facts.

All humans have an area of the brain known as the reptilian brain

In some humans, especially the ancient intact bloodlines, this area of the brain did not evolve as the majority of human brains did, because they always breed with their own bloodlines and one result is that they have over developed reptilian brain areas.

It's only a theory, based in part on the concept of the Triune Brain Region.

https://en.wikipedia.org/wiki/Triune_brain

The reptilian complex (also known as the "R-complex", "reptilian brain" or "lizard brain") was the name MacLean gave to the basal ganglia, structures derived from the floor of the forebrain during development. The term derives from the idea that comparative neuroanatomists once believed that the forebrains of reptiles and birds were dominated by these structures. MacLean proposed that the reptilian complex was responsible for

species-typical instinctual behaviours involved in aggression, dominance, territoriality, and ritual displays.

which may also explain their inhuman behaviour.