There are two powerful forces in this play that lead to evil: the witches and Macbeth’s ambition.
Macbeth probably never would have done any of it if he had not met the witches. The witches brought his ambition to a boiling point. Before they told him he might be king, it never occurred to him to be king. Granted, Macbeth definitely had it in him before he met the witches. They brought it out. Macbeth did wiggle a little on the hook, but it did not take him long to decide he was going to be king, and he was going to do anything he could to get there and then stay there.