How are we to understand what Hecate says toward the end of Scene 5? "He shall spurn fate, scorn death, and bear his hopes above wisdom, grace..."
In this scene, Hecate expresses her displeasure with the Weird Sisters as a result of their meddling with Macbeth without inviting her to join them and for no real purpose. She makes plans to meet up with the three witches in the morning but "This night," she says, "I'll spend / Unto a dismal and a fatal end" (3.5.20-21). Hecate is making plans for what to do with Macbeth: she claims that she is going to catch a droplet from the moon and work some magic with it to conjure up spirits to trick Macbeth and confuse him. She then claims that
He shall spurn fate, scorn death, and bear
His hopes 'bove wisdom, grace, and fear.
And you all know, security
Is mortals' chiefest enemy. (3.5.30-33)
In other words, these confusing spirits will make Macbeth feel so confident in his success that he will ignore fate and scoff at death, guided only by his hopes rather than by wisdom or caution. He will have no doubt of his future success, and this will make him relax. Because the apparitions' messages will make him feel secure, he will let down his guard and become vulnerable.
In this passage, Hecate is saying that she will conjure a vision that makes Macbeth overconfident, and thus help push him toward his destruction. The quotation ends with Hecate's prediction that Macbeth will "bear his hopes above wisdom, grace, and fear," meaning that he will believe against all evidence that he will prevail over his enemies. This will make him pursue a self-destructive course of action. Her prediction comes true when Macbeth visits. He sees three visions, and from them gets the prediction that he can only be killed by a man "not of woman born," and then only when Birnam Wood (a forest) marches on the castle of Dunsinane. Her final observation that "security is mortals' greatest enemy" proves prophetic.