Maybe because of their survival instinct and strictly following their traditions.
It's more likely that they haven't found any cure to their curse(?) and they want everyone in their clan to participate and be obedient otherwise it'll be the cause of their own wipe out.
But to think they would kill those who don't follow. Tsk tsk.
They are literally their own destruction.. how is that protecting their clan when they're the one doing all the killing work? Tch tch, indeed.
Yeah. But like the emperor's brother said, if they get sexual intercourse and have a child with 'normal' humans then it'll be suicide. And If one is forgotten then it's death too so it might also apply within each clan. Idk sounds stupid and cruel but it's like the only choice they have.
Their traditions are double edge sword, is what i mean
Having the same dna can cause problem to the baby of those Aerhi(I forgot the clan name) siblings.. it can lead to different malfunction to the body of that child I think..
Maybe that's their unique characteristics as well, who knows.
But man, why is that clan so cruel with their own blood relates?