What moral or other conflicts between different characters and value systems bring about the tragic end of Le Morte d' Arthur, both between different characters and their different value systems?
First there is a love triangle between Arthur, Guinevere, and Lancelot. Lancelot is one of the knights of the round table and first, his loyalty should be to the king and his duty; however, he cannot resist his love for Guinevere, who is Arthur's wife and queen, and he betrays Arthur to be with her. Guinevere is torn between her love for Arthur (and sense of duty) and her love for Lancelot (and intense passion). Arthur obviously feels betrayed by both his wife and Lancelot...he loves his wife dearly and trusted Lancelot as one of his knights.
Obviously, you have the "value system" of the knights of the round table. Their code was one of honor and duty and allegiance to King Arthur. They were the elite of all of the nights and were closest to King Arthur. Because Lancelot was one of these knights, his betrayal is particularly disturbing for Arthur. Also, you have individual value systems that are held by each person (Guinevere, Arthur, and Lancelot). All 3 are torn between their senses of duty and the intense love they share for each other.