Let’s call him/her “Pat.”
Pat is the kind of friend who butters you up a lot, so you end up telling Pat about every little quirk and insecurity of yours in full trust. You eventually learn that Pat has used those admissions for “material,” dishing you to everyone else Pat can track down–not only breaking your confidence but painting you in the harshest light possible.
But at the same time, Pat’s filling you in on all the entertaining flaws and peccadillos of Pat’s other friends, making it appear as if he/she thinks they’re absolutely psycho (though to their faces, Pat’s quite solicitous, of course, and wants to string the whole bunch along onto a pretty necklace possessed only by Pat.)
As a result of the relentless poisoning, you wind up hating all of Pat’s friends and all of Pat’s friends end up hating you right back. And Pat remains the king/queen of the scene.
I had a friend like this in high school.