Macbeth was always going to involve intrigue and misunderstanding as "fair is foul and foul is fair." (I.i.10)The witches play a crucial role in the development of Macbeth's character and how he uses their prophesies to further his "vaulting ambition."(I.vii.27)
In recognizing that the witches "cannot be ill; cannot be good" (I.iii.131), Macbeth reveals his excitement at the accuracy of their first prediction. He knows that "nothing is but what is not"(141) and is further motivated by Lady Macbeth's involvement; albeit bullying. Macbeth really just needs Lady Macbeth to reassure him, which she does and "a little water clears us of this deed." (II.ii.67) macbeth's biggest fear is of being caught.
After Duncan's murder Macbeth is initially troubled and his guilt shows in his confusion when he brings the daggers with him - almost incriminating himself. Lady Macbeth saves him in that instance. After Banquo's death, Macbeth is haunted by Banquo's ghost and again Lady Macbeth allays his fears; even though she was not involved in the plot to kill Banquo.
Macbeth's character is thus developing without Lady Macbeth and he needs to visit the witches again to give him encouragement to continue in his quest to be king. Act IV.i. is the culmination of all his desires as "none of woman born shall harm Macbeth"(IV.i.80) and his words at the end of the scene reveal his overconfidence and self-absorption as "this deed I'll do before this purpose cool." (154 )Macbeth's ability to be rational escapes him although he knows that "damn'd all those that trust them." (139) yet he still intends to murder innocent people - "all unfortunate souls that trace him in his line."(153)
The fact that the witches have vanished and that Macbeth is so restless to press on reveal his continuing descent into madness; albeit more subtle than the doomed Lady Macbeth who "needs more the divine than the Physician."(V.i.72)