At the beginning of the play, Macbeth was an honorable hero, a normal human being with a normal mind. He had killed Macdonwald in battle, but that was for King Duncan. No one would have ever expected him to turn into a tragic hero "a character that suffers flaws leading to their downfall", but by Act 1, Scene 3 that is just what happens.
In Act 1, Scene 3, the weird sisters meet Macbeth and tell him that he is the thane of Glamis, thane of Cawdor and that he "shalt be king hereafter." (Shakespeare, 276) Once he is told this, he figures that if he is going to be king, then he must kill the current king, Duncan, to get the throne himself. Before he heard the weird sisters prophesies, he was always honorable and loyal to Duncan, and I don't think he would ever have had a thought of murdering him. I think that the witches had major influence on him that day.
Once again Macbeth meets the witches in Act 4, Scene 1. This time, they used apparitions to get their message through. They were in the form of an armored head, a bloody child, and a crowned child. Each had a message to tell Macbeth. "Macbeth! Macbeth! Beware Macduff. Beware the thane of Fife!" (Shakespeare, 323) After hearing these messages, he sends murderers out to slaughter lady Macduff and her son. The witches are in part responsible for this murder as well, because they send out messages to him and plague his mind with these thoughts, and therefore influenced him to do this horrible task.
The only murder that I feel Macbeth was not influenced to do was Banquo's murder. The witches only speak of Banquo when Macbeth meets with them for the second time and Act 4, Scene 1, when Banquo had already been killed. (Witches and the Influence on Macbeth, 1) The only reason Macbeth killed Banquo was because he had witnessed him killing Duncan, and didn't want anyone that might tell on him around.
I believe that if Macbeth chose not to listen to the Witches, that he would have been much happier with his life.