Malignant narcissists will use any and all information against you. Say, for instance, you're hoping for a promotion at work. You happen to share this desire with someone in your office, who has strong narcissistic traits. You also let her know this promotion, more or less, hinges on the success of your upcoming project.
A normal person will wish you well and sincerely mean it. A malignant narcissist, on the other hand, sees your success as an affront to her very being. Gripped by insane envy, she will do everything in her power to try to make your project flop. This can range from spreading nasty rumors, "forgetting" to let you know about an important meeting (critical to the project) or even trying to organize a lot of social events outside of work, in an effort to distract you from focusing on the project. The night before the big presentation she arranges an office meeting you can't afford to miss. It stretches late into the night. Then, it's followed up by drinks at a local cafe. This is all designed to make you so tired that you can hardly see straight the next day.
People who've never been involved with a female malignant narcissist may have a hard time believing someone could be so conniving. However, those of us who've crossed paths with one of these characters know they're capable of everything described above, and a lot more as well.
This is why we need to keep our personal information tightly under wraps, especially at work.