There is certainly evidence in the play to support such a claim as this: Macbeth’s descent from a celebrated warrior into a treacherous individual is brought about by external forces. Initially, Macbeth is revered as a "brave" and loyal servant of the crown (1.2.18). A Captain in the Scots forces reports that Macbeth fought as though he "Disdain[ed] fortune" and acted "Like Valor's minion" when he fought valiantly against the rebel, Macdonwald's, forces (1.2.19, 21). Duncan, Macbeth's king and also his cousin, calls him a "worthy gentleman" and rewards him handsomely for his service (1.2.26). Lady Macbeth, Macbeth's own wife, also thinks of his very nature as being "full o' th' milk of human kindness" (1.5.17).

It is only when the vindictive and cruel Weird Sisters determine to "meet with Macbeth," telling him that he is the Thane of Cawdor and will be king, that Macbeth's ambition begins to overrun his conscience (1.1.8). When he learns that he has been named Thane of Cawdor, it seems to him like the Weird Sisters predicted the future. However, they were only reporting something he did not know yet. He begins to hope, then, that he will become king too. When Duncan names Malcolm the heir to the throne instead, Macbeth begins to consider his own "black and deep desires" (1.4.58).

It is not hard to see that Macbeth does not begin to consider anything untoward or cruel until the Weird Sisters influence him with their prophetic-seeming statements. In fact, two of the people who would know him best in the world think of him think of him as a loyal and righteous person.

The Weird Sisters, on the other hand, seem only to have bad intentions in their dealings with other people: when a woman denies the First Witch some of her chestnuts, the witch determines to follow the woman's husband and torture him by preventing him from sleeping (1.3.20-21). She also has a random "pilot's thumb" on her person (1.3.29). These are not good people. They enjoy wreaking havoc, and so it seems likely that Banquo is right about them when he opines that they are "instruments of darkness" who have told him and Macbeth "honest trifles" in order to betray them "in deepest consequence" (1.3.136, 137, 138). Macbeth only begins to do bad things after the sisters interfere with him.