Was Duncan naive to trust Macbeth in Macbeth?
Duncan was not naïve to trust Macbeth, because Macbeth had been a loyal and trusted subordinate and cousin until the day the witches told him the prophecies.
It is true that the first Thane of Cawdor was a traitor. Perhaps it was time to retire that title! However, there is no evidence whatsoever that Macbeth had done anything untrustworthy up to that point. On the contrary, he had just shown his bravery and loyalty in battle.
In Act 1, Scene 2, the bloody sergeant tells tales of Macbeth’s courage and skill in battle.
For brave Macbeth—well he deserves that name—
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,(20)
Like valor's minion carved out his passage
Till he faced the slave… (Act 1, Scene 2, enotes etext pdf p. 9).
It is for this bravery, including fighting his way through a line of soldiers to get to the traitor Macdonwald, that Macbeth was promoted. Apparently when you kill a guy, you get a title.
Duncan actually apologizes to Macbeth for not promoting him sooner.
O worthiest cousin!
The sin of my ingratitude even now
Was heavy on me. (Act 1, Scene 4, p. 17)
Macbeth even comments himself that Duncan has no reason not to trust him.
He's here in double trust:
First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host,
Who should against his murderer shut the door .. (Act 1, Scene 7, p. 22)
In fact, it is because Duncan is so trusting and has recently honored Macbeth that he decides he does not really want to kill him. Lady Macbeth talks him into it.
There was absoultely no reason for Macbeth to kill Duncan, or for Duncan to even suspect he would want to be king. He was not next in line, or even third in line, for the throne.