Fiends are more interested in turning good mortals to evil because their souls have more value and power than beings who are already evil. Wyll, for instance, seems like a good person, being a monster hunter and a heroic 'Blade of the Frontier' and his deal with a devil must have been made with good intentions, but we all know where the road of good intentions leads. Ask Asmodeus, Zariel and so many Devils who were once Celestials. I imagine that whoever Wyll's Patron is, wants him to slowly succumb to evil to make his soul all the more valuable. Wyll seems to regret his pact and he wants to get out of it, that is not the action of a man who is committed to evil. Although based on the blurb, Wyll's patron seems to be in a bit of bother, so he might have a chance to renegotiate the terms of his pact.
