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.)
Blackmailing isn't a deterrent? Sure, it's "more content," but I never found a player who enjoyed sacrificing money or XP or skill points (whatever he accepts as payment) for no reason. Sure, the first case of the demons attacking you for devouring three souls might be less of a sacrifice, but that was just an example. The point is, you can have both tangible and roleplay consequences for devouring souls packaged into one thing, instead of a mechanical deterrent, which to me, is just as gamey as a smile-sad face karma measurement system.
I think 8-12 situations, some part of the main quest, where obtaining source points in an evil way has direct, negative consequences would be good as a form of karma. Your average person shouldn't be able to see the evil things you've done, but powerful wizards, other sorcerers and spirits might. You'll lose out on rewards, get in fights you wouldn't have as a good sourcerer, suffer mechanical consequences, and suffer from negative roleplay situations. Say, as an evil sourcerer, you accidentally corrupt a malleable spirit (say, it's sensitive to your evil aura or something) and it goes and kills what was once its family. Also, you should definitely face harder fights with the main antagonists, who hate source because they think its evil. You're proving them right, and they'll give you all they got. But if you're good, their motivation will be slightly lessened. You might even able to convince some source hunters that you're good and that the inquisitor is the evil one.
And yes, I think these situations ultimately might have to be tied to a count of how many souls you've devoured. The player shouldn't know exactly how many devoured souls lead to each outcome, but they should know that 10 devoured souls will probably get you to worse situations than 3 devoured souls. And you can probably do something like certain souls are more evil to devour -- for example, children, and others don't even count.
Anyway, that's my perspective. Maybe some sort of mechanical drain effect, or a sort of addiction thing, but I'd prefer more integrated downsides.
