The tragic downfall of Macbeth was ultimately his fault. Though other influences contributed also. It was combination of three dark forces: supernatural, external, and internal. The three witches and dark powers behind them represent supernatural forces. Lady Macbeth is the outer force that pushes Macbeth towards the bloody deeds. Macbeth's own ambition acts as deciding power in bringing him to his downfall. .
The witches and their prophecies are the first major influence on Macbeth's actions, but their influence is only that of suggestion. Their power has limits, as shown in the first scene of the play. One witch was refused chestnuts from a sailor's wife, they directed their revenge at her husband, but were unable to kill him, only make is life at sea hell by influencing the elements. "Thou his bark cannot be lost, yet it shall be temptest-tost" (I, iii).
The witches prophecies don't say when or how, and their meaning depends entirely upon the receiver's interpretation. When the witches tell him he will be King Macbeth's reaction is one of shock then a wish to know more. "He seems rapt withal / stay you imperfect speakers, tell me more" (I, iii). Stark contrast to Banquo who laughs his prophecies off as jokes. "Have we eaten on the insane root" (I, iii).
Macbeth almost immediately begins to consider murdering Duncan. "If good, why do I yield to that suggestion, whose horrid image doth unfix my hair, my thought, whose murder yet is but fantastical" (I, iii). And immediately Macbeth writes to Lady Macbeth, "They met me in the day of success; and I have learned by the perfectest report, they have more in them than mortal knowledge." (I, v). He obviously has great faith in the witches' words probably because of the success of the first two. "Two truths were told, as happy prologues to the swelling act of the imperial theme." (I, iii). .
Later on, when thirsting for greater knowledge Macbeth encounters three apparitions, called by the witches.