This is probably wickedly wrong, but that would hardly be out of character. Consider the Infernal Flaw: Corrupted Abilities. This Flaw, representing infernally tainted training, gives a character a +3 to all immoral uses of the selected abilities; it may cover as many abilities as one likes. If the bonus causes success, the character gains 5 xp in the ability.
Now, if I were an infernal character, I would probably have some infernal methods and powers. But I would certainly have learned them from an infernally tainted source, as how could I not? Thus it makes perfect sense to take this flaw and apply it to my infernal abilities. But every use of those ailities is immoral, which means that every such use gets a +3 bonus (per ability, for a +6 bonus in most cases), and that many such uses will earn an additional 5 xp. This is an extreme fast track to tremendous power, since you can just use the ability for the immoral purpose of becoming more infernally powerful. And even if we don't let in that formality, you can still use the abilities very, very often.
Should this be permitted, do you think? If so, should it be limited, so that the immorality of the act needs to be greater the better you are at the relevant ability before you gain the bonus? By the time you're at, say, level 10, your acts would have to be so wicked and repugnant as to horrify the Marquis de Sade, and Satan himself would blanch at a level 12 evil....