Although the witches were a main cause of Macbeth's downfall, the blame cannot lie solely with them. Rather, the unfortunate events were the result of poor judgement, human weakness, and adverse circumstances. The three "weird sisters", as they are referred to in the play, planted a seed in Macbeths mind. This seed was the idea he could become king. They did this by saying: "all hail Macbeth, hail to thee Thane of Glamis." then "all hail Macbeth, hail to thee Thane of Cawdor." and finally "all hail Macbeth, that shalt be King hereafter." Macbeth was already Thane of Glamis...
his bravery and valour in battle. However, when it came to strength of character Macbeth was lacking. He let his wife make his earlier decisions for him by manipulating him with her words. He did not have enough strength of mind to make his own decisions about the actions he was about to commit.

Although the witches were a very large contributing factor to the downfall of Macbeth, they weren't the only one. Had Macbeth not wanted to be king or lady Macbeth not have pushed him towards it then the outcome could have been somewhat different.