Final answer:
King Duncan from Shakespeare's Macbeth is best described as gullible due to his misplaced absolute trust in individuals who eventually betray him.
Step-by-step explanation:
The negative description one could associate with King Duncan in William Shakespeare's Macbeth among the options of being two-faced, insane, gullible, or indecisive is gullible. This is evident in the line spoken by Duncan, "He was a gentleman on whom I built an absolute trust." He shows a lack of discernment regarding the true nature of the Thane of Cawdor and later Macbeth, who ultimately betrays him. Despite signs of loyalty, Duncan fails to see through the deceit, making him susceptible to betrayal, highlighting his gullibility.