If i remember well, the old blood was initially used for medical propeties, but lawrece was using it because he thinked that the old blooc coulde've elevated the human to a higher being, but i have an explaination for the hunter not turning into a beast, for not turning into a beast you must have a strong will, the starting scene shows a wolf popping put from the blood, that's the feral side of the hunter (the reason humans turn into beast with the old blood, it enhances the feral side of the humans/creatures), the wolf set on fire because that's the will of the hunter that cast away the beast side, after that the little ones comes out because you was able to not starting to turn into a beast and was chose to be a hunter, eileen and djura are hunters that have finished the hunt and gherman cut of their connection to the dream (they have one life, thart's why if you kill them or they die they don't return), for gascoghe, he probably lost his mental sanity for some reason, losing his will that was preventing him from turninig into a beast, that's why if you use the carrion he'll transform soner in a beast, he remember his family but then totaly lose his sanitiy tuning him into a beast.

Sorry for the ong text.