this is shown in act 1 scene 3 the evidence is on line 50 is says First Witch All hail, Macbeth! hail to thee, thane of Glamis! 50
Second Witch All hail, Macbeth, hail to thee, thane of Cawdor!
Third Witch All hail, Macbeth, thou shalt be king hereafter!

that’s why i think the witches control his fiat because all of it happened the witches were prophecies to elaborate on my point they said it’s like hes already is king and thane of cawdor etc. and he got so happy so i think macbeth is not in control of his fiat and the old thane of cawdor died because of cawdor and as he quoted yes im thane of cawdor