Macbeth is a man who wants something he cannot properly have.
If he was ever to become King legitimately, (had he refrained from his crime), maybe his “grapes would have ripened.” But as it stood in this story, they turned out to be as sour as his reign.
If grapes can stand as a metaphor for attaining the crown, the mischievous witches were dangling sour grapes in front of a starving foxe’s face to mislead him into thinking there was a satisfying snack in store; unfortunately the fox couldn’t distinguish between ripe and unripe ones; still jumping for them, he ate them and was shown that they were nasty, sour & unsatisfying.
As he was distracted trying to swallow these gross grapes which struggled to go down his throat, he fell into a trap and eventually starved.
Figuratively speaking, this is what happened to Macbeth; what Macbeth saw looked like tasty fruit; it just didn’t turn out to be. He might have become a great king and certainly had hope of that, but it wasn’t his time yet. Macbeth embraced enthusiastically his future as King but ultimately failed to acknowledge the fact that he was abundantly lost in delusion and in a state of utter “unripeness.”
After falling for the witches deceitful promises, Macbeth embraces the idea of attaining something he cannot properly have. He was tricked into procuring his own downfall through the stimulation of ambition for something out of his reach; The Weird Sisters knew his credulity for matters superstitious as a “foregone conclusion,” & from this he wasn’t level-headed enough to eschew it.
Naive and subsequently evil; just plain evil.