Devour 3 souls, and the first demon arrives to devour YOUR soul. Devour five, and another one tries to blackmail you (credit to Lacrymas for that idea). But I don't think anything should make up for devouring souls. You devour five souls, and it doesn't matter if you've been good the whole rest of the game, that demon is still going to blackmail you.
The problem with that kind of system is that it's not really much of a deterrent. Even if the events and mobs are set to give no XP at all, you effectively get
more gameplay out of it, and no lasting disadvantages. The only system I can see that really works as a counterbalance to the rather marked benefits of devouring souls to gain Source points, is to have low karma have some sort of direct, mechanical effect on your character---preferably something that comes into play in combat, seeing as there will be a number of players who will more or less ignore the social and role-playing aspects of the game. Magic use would be the obvious choice, seeing as it is directly connected to the character's soul.
(There is more deliberation on how it could be done
here.)
