Something like that changes the whole anime and story.
If Teresa killed Pracilla, Clare would have no reason to turn into a claymore.
Perhaps she would have just continued to travel with Teresa?
Or if Teresa and Pracilla both awakened, then Clare wouldn't have teresa's blood in her.
Her chances of dying while in the presence of both of them would be higher.
If she would survive, she still might not choose to become a claymore?
Because teresa would just be awakened, not dead. So there's no need to get revenge?
Well, yeah, it changes everything.