[@Fabulous Knight]

Ah, but I'm still left wondering,

[quote] At what point would he stop granting powers to a necromancer/demon worshiper if that one was utterly psychopathic and more than willing to do or give anything for power?[/quote]

To take your example, what if Ashmedai gratefully takes the power and continues to offer sacrifice or whatever he agreed to do in order to gain his abilities, and then proceeds to light the world on fire and kill everything for his own amusement? Does Mammon then come in and test his resolve by saying that he can't do such things, rendering Ashmedai's horrible powers essentially useless for the purpose that he had envisioned? What if Ashmedai now ignores Mammon and tries to keep using what he was gifted?