Question about the end of Unholy Consult

So at the end of the book it's revealed that the Consult, during the process of wiping out the Dunyain, actually managed to capture a few alive and bring them back to Golgotterath. Unsurprisingly, these Dunyain then gained control it.

My question is...why would the Consult even take Dunyain prisoners at all? They're well aware of the the ability of the Dunyain to manipulate people by observing Kellhus and his take over of the three seas. They're a major threat, and the Consult pretty much had them at their mercy, what reason would they need to keep some alive?

Unrelated question but one I've been wondering about since I first started reading the series...why would anyone become a sorcerer when they know they are damned because of it? Doesn't seem like any amount of power would be worth spending eternity in hell.