izdajnik

1. traitor; betrayer
   Nazvali su ga izdajnikom jer je prešao u drugu stranku. — They called him a traitor because he switched to another party.
   U grupi se krije izdajnik. — There is a traitor hiding in the group.

The feminine is izdajnica. The synonym izdajica declines like an -a noun but refers to either a man or a woman; when it refers to a man, masculine agreement is usual: taj izdajica.