Choice and consequence for specific actions is nice, but it's helpful to quantify the effect of more generic crimes and good deeds, particularly for devouring souls or the like. I think the best way to use a generic karma system is to make it most prominent in the hall of echoes, where certain good or evil spirits will arrive or give you offers based on general trend of acquiring source points. Probably you just need certain thresholds for certain beings to arrive. 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.
Probably best to keep the system under the table instead of giving you some kind of karma bar. You'll just notice demons showing up sometimes, and you won't really figure out the thresholds until your second or third playthrough.
