In Macbeth, the witches have a devastating influence on Macbeth. In Shakespeare's day, the witches or "Weird Sisters" would have been the epitome of evil and, in terms of the witches own words in Act I, scene i, line 10 when "Fair is foul, and foul is fair," they would have roused the audience's feelings of mistrust and contempt, foreshadowing what will follow.
Deciding which decisions appear rational and which are irrational depends on the intention. The witches know how susceptible Macbeth is and so should have anticipated a reaction but they cannot be blamed for his "vaulting ambition." And while the witches' decision to tell Macbeth that he would be king is truly irrational and plants, by Macbeth's own admittance, a "horrid image" (I.iii.135) in his mind, it is not their decision to tell him which causes his ultimate downfall but his own decision to take matters into his own hands.
Having made the decision to inform Macbeth of his future title as king, the witches relish Banquo's questions and give him information which makes him wonder if he has gone crazy as he says, "Have we eaten on the insane root?" (84). They disappear before they can be questioned further which could be considered a good decision because Macbeth and Banquo cannot necessarily act upon this information. Banquo accepts the information but makes no devious or deadly plans, unlike Macbeth who, without guidance, turns to Lady Macbeth to plot their murderous acts.
Later, when the witches reappear, they are scolded by Hecate who knows that Macbeth is "A wayward son...spiteful and wrathful...who...Loves for his own ends" (III,v,11-13). Hecate wants to teach Macbeth a lesson and the witches put together a spell which will affect Macbeth most deeply because he is, by nature, "wicked."
This should teach Macbeth that his vanity, his grandeur and his murderous deeds have served no purpose and that he must beware especially when, after the third apparition, he is feeling most confident; Banquo's line still appears and will apparently usurp his crown. Therefore, the decision to warn him so spectacularly and then to reveal to him that he still cannot change the future because Banquo's heir's will ascend the throne could be seen as a rational decision because any rational person should realize that he cannot win, even if he cannot be beaten by one "of woman born" (80) or until "Birnam wood...come against him" (93). Macbeth should stop trying but he is far too intense and unreasonable to understand the dire warning. To him it is a challenge, not good counsel.