Final answer:
Sir Mordred used the affair between Lancelot and Guinevere to disrupt the round table and bring about the downfall of King Arthur.
Step-by-step explanation:
The individual who used the sin of Lancelot and Guinevere to disrupt the round table for traitorous purposes was Sir Mordred. Mordred was often depicted as a nemesis of King Arthur, and in various Arthurian legends, he took advantage of Lancelot and Guinevere's affair to cause dissent among the knights and to ultimately bring about the downfall of King Arthur's reign. The resulting conflict culminated in a battle where both Mordred and Arthur were fatally wounded.