I've read accounts, written by sociopaths, that early in life they realize they are different. So they learn to mimic the appropriate emotional responses, in order to blend in with the rest of us. Early in a relationship, while a malignant narcissist is sizing you up, she'll be anything you want her to be. If you have a deep love for animals, so will she. If you tell her you are passionate about feeding your family organic food, she'll tell you she feels the same way, and that's what she put on her table each night as well. (However, you also notice she rarely cooks and that her husband and children seem to subsist on frozen pizza.)
If you care deeply about your children, expect to hear about all the time she spends with her family, in an attempt to portray herself as a devoted mother. (Strangely enough, though, her children are usually at your house because she's so busy running here, there and everywhere.)
Malignant narcissists are chameleons, because they change personas quickly. This is one reason why they usually prefer interacting one-on-one, as it's too difficult to undergo a rapid change of masks. That's because they need to wear a different one for each person they're speaking with.
This need to find a way to blend in has probably contributed to their seemingly superhuman ability to read other people and to ferret out "useful" bits of personal information, which serves them well when they decide to attack and destroy their former "friends."
These qualities are also what make malignant personalities so dangerous. Because they mirror our actions so well, we tend to trust them. Then, once they gain our confidence, we confide in them. Then, all of this information can and will be used against us. Narcissists are also very persuasive, so most people believe everything they say, even if it happens to be an outrageous lie.