just wanna post my opinion on this,
fate (currently) probly a puppet for someone (the real fate, maybe??) cos on the reencounter, he'd used the swoshing liquid to evade attacked by ?? (man i've forgot who was it).
anyway, the theory that nagi and negi decended from demons maybe plausible cos IMO its more on ancestry level of sorts as nagi is slacker that fail his studies yet like eva'd said he had the power to use high level magic on here (more or less).