King Duncan's response to MacDonwald's death, "There's no art to find the mind's construction in the face. He was a gentleman on whom I built an absolute trust."
How could this be a dangerouse thing for Duncan
Duncan's statement in Act I scene 4 is dangerous because it introduces the king as a leader who is not a good judge of character. MacDonwald, the previous Thane of Cawdor, died a traitor to his king and country, but before he was killed, he was a man in which Duncan held in high regard.
Duncan places the same trust in Macbeth, the new Thane of Cawdor. Just as with MacDonwald, Duncan cannot "find the mind's construction in the face" or in modern terms, you cannot trust a man simply by looking at his face. Macbeth's face is that of a loyal servant and Scotsman, but the desires he is hiding are those of being king one day himself.