Everything looks so promising for Macbeth. He is a valiant soldier, courageous in battle and respected by his king. We know that things are not as they seem when Macbeth comments on the "foul and fair a day" (I.iii.38) and he and Banquo are intrigued by the witches. The witches hail Macbeth who knows that for him, being king, such as they suggest "stands not within the prospect of belief."(73)
When Macbeth is told that he will be Thane of Cawdor and rather than recognizing the connection to the battles won, he is already wondering about the prospect of kingship and questions Banquo as the witches "promis'd no less to them." (119)(his sons). Banquo does caution Macbeth as "the instruments of darkness...betray's in deepest consequence." (125)
Macbeth is still rational and his admittance that the witches "cannot be ill; cannot be good" (I.iii.131) doesn't stop his already "vaulting ambition" from overtaking itself as he considers why the witches would tell him truths - such as being Thane of Cawdor - and then deny him the rest of their prophesies as "nothing is but what is not."(141). It is disturbing that Macbeth is reading so much into the present circumstances even to the point when he takes his leave from Duncan so as to notify Lady Macbeth and he recognizes the potential for failure as The Prince of Cumberland, Duncan's son " in my way it lies."(I.iv.50)
Up until this point Macbeth could just be letting his excitement and ambition cloud his judgement as he has attained this esteemed title through his military prowess and his devotion to king and country. He even questions his own motives and realizes that what he is proposing "o'er leaps itself" (I.vii.26)
Macbeth is influenced by Lady Macbeth as she suggests that he could "be so much more the man" (51) when she realizes that his determination is faltering. The decision to "proceed no further in this business" (32)is, thus, soon forgotten and Macbeth barely looks back save to worry that he may be caught as he strives to ensure that " false face must hide what the false heart doth know."(82)
Hence, the influence of Lady Macbeth, the witches, circumstance and his own ambition combine to ensure Macbeth becomes the epitome of evil. Macbeth's misdirected fear of being caught which he mistakes for guilt, sets Macbeth on the road to his own self-destruction.