You raise such an interesting point John let me quote from an established academic "
Greek mythology begins and ends with tales of patricide, symbolising perhaps, the cycle of father-to-son power hierarchies and the ultimate futility of these struggles against the pre-woven patterns of fate. The son-kills-father template is laid out in Greek myth in one of its earliest origin stories, where the titan Kronos is warned by mother Earth (Gaia) that one of his sons is fated to usurp him. Hasty to sidestep the path of destiny, Kronos devours all twelve of his children as they are born- except of course for the youngest- Zeus, who is saved by his mother and eventually returns to indeed supplant his Dad. Perhaps Kronos deserved it- he after all, committed the same crime against his own father Ouranos, but with the added insult of castration.
The parallels between this myth and the characters Macbeth and Duncan are clear, with the exception that it is the son-figure, not the father-figure, who receives the Delphic promise to ‘be king hereafter!’ (I.III) The relationship between Macbeth and Duncan is established at the beginning of the play as a familial one. In the first act, Duncan greets Macbeth as ‘valiant cousin!’ and his praise for Macbeth’s battle prowess rings with hyperbolic, fatherly pride, rather than kingly entitlement"