Why doesn't King Arthur trust Mordred? Mordred is one of the knights of the Round Table. Mordred wants Arthur to sign a peace treaty. Mordred tries to steal the English throne. Mordred tries to steal the magic sword from Arthur.

Initially, King Arthur did trust him, at least enough to make Regent over Britain while he went to Gaul. His trust was lost when Mordred tried to marry Guinevere and steal the English throne. Mordred claimed that his marrying Guinevere was consensual, but as it turns out she fled when he proposed.