trusted ally, or former-partner.
2) Back-stabbed by a person who has always portrayed
themselves to you as a loyal and trusted friend.
From the play "Hamlet", in which Prince Hamlet's boyhood friends Rosencrantz & Guildenstern pretend to be loyal to Hamlet so they could spy on him and report back to the treasonous new King (Hamlet’s uncle, who slept with his mother, killed his father, and stole the thrown of Denmark).
Just like in the play (spoiler-alert) you should have anyone who Rosencrantzes you killed when the time is right. People who Rosencrantz their most trusted friends will betray anyone and everyone eventually; they have no loyalty, no pride, and thus no place among men nor any business in a profession of honor. They must all be put down for the good of society.
*Not to be confused with Ratted-out or Snitched-upon; you should expect a Snitch or Rat to turn on you eventually, but to be truly Rosencrantzed requires betrayal by a seemingly loyal friend you have a long, trusting history with.