You can read a Shakespearean play over and over and the fun of it is in the re-reading of the play from different persepctives. Elizabethans did believe in witchcraft and witches were considered to be evil, and so to an extent they did contribute to the whole aura of evil. However... You cannot make a person do something if the idea wasn't already there in his/her mind. Macbeth already had the thought in his head of killing Duncan, so he was easily led astray. By the second time Macbeth meets with the witches you can see him becoming more arrogant... he doesn't really need them...The greatest evil in the play is in the man himself. He is the one who kills a king and gets others to commit crimes. Yes, I believe the witches add to the whole idea of evil, but Macbeth, the man, plays a much more significant role as a person capable of heinous crimes.
Yes, certainly the witches, whenever they appear in the play, suggest a very strong image of evil because they appear unnatural. Lightening and thunder introduce them in 1.1, and their even their words portend evil, turning meaning upside down: "Fair is foul, and foul is fair, / Hover through the fog and filthy air" (1.1.11-12). After their first conversation with Macbeth, Banquo says "the earth hath bubbles . . . And these are of them (1.3. 79), suggesting something unnatural. Act 2.3 also uses images of the unnatural, showing nature revolting against itself in protest of the imminent murder of Duncan. Lennox says, "The night has been unruly. . .Lamentings heard i' th' air; strange screams of death,/....Some say, the earth was feverous, and did shake" (55-61).
The Witches or the Weird Sisters certainly contribute to the evil atmosphere in the play. But not in its entirety.
The Elizabethans always believed in the existence of the Witches and therefore, when Shakeseare wrote the play, he was only keeping in mind the requirements of his audience, like any other playwright.
However, when we come to think of it in a different perpective, the actions that the witches caused were indeed, quite evil. The prohpecies made by the Witches accelerated an already scheming-mind in Macbeth, and led him to do what ultimately kills him in the end. Moreover, we must also understand that you cannot make a person do something, if he hasnt already been thinking about doing it sub-consciously. Macbeth obviously had very big dreams, that of being the King of Scotland one day, which is why when he heard the Witches prophecies, he saw an outlet from which his dreams could actually come true. This, along with the deep influence of his unscrupolous wife, led him to the timely murders, ths earning him the name of a 'tyrant' and a 'dwarfish theif' who is unable to fit into a 'giat's robe' by his once-adoring and admiring public.
There are several other factors that contribute to the evil facets in the play. The atmosphere itself, was quite dark, gloomy and mysterious. The daylight did not bring any new hope, and continued to contribute to the gloomy theme of the play. The unnatural incidents that occur, may also be considered reponsible.