Vesper Lynd is the Treasury liaison assigned to work with James Bond on the Casino Royale mission. She is later revealed to be a double agent working for Russia's SMERSH. While it is never explicitly stated, it is heavily implied that Vesper betrayed Bond and caused his capture by SMERSH.
While Vesper's motives for betraying Bond are never fully explained, it is speculated that she was blackmailed by SMERSH into working for them. It is also possible that she simply saw an opportunity to further her own career by aligning herself with a powerful organization like SMERSH.
Whatever her reasons, Vesper Lynd is undoubtedly a traitor. She betrayed Bond's trust and put him in grave danger. She is responsible for the death of many innocent people, including Bond's love interest, Vesper. Vesper Lynd is a cold-hearted traitor who deserves to be punished for her crimes.