Nemesis's power to influence people is limited. It can't bend them into something totally opposite of what they were. From what we've seen of it so far, it removes some of your natural restrictions, gives you "free" will, and nudges you down roads where your desire or ambition is already leading, but common sense or the order of things keeps you from pursuing. I don't have the book on me, but Nemesis is cited in tons and tons of mortal cases of just this.
Aurora didn't become an evil Winter Fae. She wanted to do something fundamentally inline with her character: end suffering, out of kindness.
Maeve didn't become kind and gentle. She remained selfish and conniving - but Nemesis spurred that selfishness to madness.
Nemesis pushed Aurora as far as she could go.
Now what I think is notable is that Nemesis does this and pushes people toward evil ends, whatever their intentions.
...and a wizard by the name of Harry Dresden also has done the same thing to the shadows of Fallen angels, Marcone, and wildfae, but for noble ends.
Harry is the anti-Nemesis.
EDIT: !!!!!!
Harry himself does this - he does crazy things and exercises free will etc. to do good/noble things (and its contagious.) That's what made him special. The doll/nature spirirt he summoned - it knew about it. If you recall, it asked him WHY he lives the way he does, because the WHY was VERY VERY important.