Reply:
I don't think purity was the reason, it was the sects secret techniques(like skill books) that they killed them for, they wanted the techniques back. And when they found out this mix technique was actually pretty powerful, they got greedy (maybe?) And wanted it for themselves OR claim it as their own as its a has part of their techniques in it.
Meh.
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I get the clan secret martial arts part. But it's pretty much hypocritical to keep the new martial arts because it's powerful, when it's obtained by blood. Might as well declare themselves unorthodox. At least they're honest in their intentions. Bet if the new combined arts is weak, they would burn it up.
It doesn't matter if it's strong or weak, their existence itself is a taboo, so much that people needed to be killed for it. It should not exist. If they allow it to be passed down, then why kill everyone from the clan in the first place? Just kill the 2 sinners and their direct descendants.
Honestly Tang clan is always morally grey compared to other orthodox sects so it's on par with their modus operandi.
I'm sorry, you guys killed the runaways, and took their martial arts? If it's already mixed with another sect's arts, why don't you just burn it? To keep your martial arts pure? Hypocrites.
Of course, this is their ancestors, not the current character's fault. But I can't with the backstory.