One of the traps many of us stroll into comes early in our relationship with a malignant narcissist. People who suffer from this personality disorder have an uncanny knack of getting us to reveal our deepest secrets. They have a number of techniques, but one of their favorites is "confiding" in us. When they do this, they seem to be genuinely sharing some highly personal part of their lives. It may be true or it may not be true, as narcissists relish telling lies. So they tell us something deeply intimate, knowing that we are not the type to use this information against them.
Because we're social beings, and we try to be decent people, when someone shares something so personal with us, we are tempted to take the bait and reciprocate, as a way of showing them that their secret is safe with us and to not be embarrassed by this disclosure.
However, whatever you tell a morally disordered person can and will be used against you. If she's already working on one target, she'll make a mental note of what she's learned about you. This will be filed away until she needs it. After she succeeds in destroying her current target, you could very well be next. The nuggets of information she's gleaned, to her, are like gold, or even better. That's because malignant personalities practically live to sow discord and ruin relationships.
The secret you fed her is the exact type of currency she needs to complete her character assassination. So be very careful sharing details of your life with people who don't know well.
Of course, a malignant narcissist doesn't need to know your secrets in order to hurt you, because she'll just make something up, if that seems to suit her purpose. But it certainly does help, as this means she also knows your weak spots. Malignant individuals hone in on your vulnerabilities in order to exploit you. These people are very dangerous, with or without information. But they are much more dangerous if they can seize upon some real dirt.
So be very careful whenever someone you've recently met attempts to "confide" in you. Don't fall for this ploy. You don't need to reciprocate with a story of your own.