As the elders of China, the Ancient Clans tried to convince the various Dynasty Legatees not to do this, as this would bring in foreign forces and harm China overall.
Originally, the fighting between the Dynasty Legatees and Great Qin had already weakened China, and now that they were allying with foreign factions to attack Great Qin, this was simply too dishonest. They were simply helping outsiders attack their own people.
Ordinarily, people would have called them traitors, as they were helping others to attack fellow Chinese people. However, they had long since stopped considering Great Qin one of them, as Great Qin had threatened them too greatly and was their greatest enemy. As such, they wanted to use any means to destroy it, whether or not it was a Chinese faction.
Moreover, in order to stand on the side of righteousness and attract even more people, they rallied people by saying, "Get rid of the evil demon and restore China's righteousness!"