In Shakespeare's Macbeth, the witches may leave the details up to Macbeth (concerning their prediction that he will be king), but they seem to know what Macbeth will do. Their intentions are evil.
The witches certainly do not benevolently try to help Macbeth out, out of the goodness of their hearts. They certainly do not tell him their predictions for Macbeth's sake. It is hard to imagine the witches as benign or neutral.
Even if one doesn't find their original predictions inherently malicious, their behaviors in Act 4.1 certainly establish their evil intent. They manipulate Macbeth, telling him half-truths (equivocating) to bring ruin to him and chaos to Scotland. Their prophesy that Macbeth can be harmed by no man born of woman cannot possibly be seen as anything but malicious. If they are not evil, why do they give him this prophesy/equivocation? Why tempt Macbeth and lead him to a false sense of security if they don't have evil intentions? Why try to fool him at all? They set Macbeth up, and when he falls for their tricks, he ensures his own downfall and condemnation.
The witches are partly grotesque, partly humorous, possibly supernatural, mysterious, androgynous, and entertaining. And they are also evil. They know exactly what they are doing, as the success of their equivocations in Act 4.1, and also in Act 1.3, demonstrate.
I do not think that the witches are a portent of evil in the play. They do seek revenge as noted in the example above from Act 1 Scene 3; however, their revenge is not personal. The sailor whom the witches shipwrecked is the husband of a woman who grossly berates the witches when she sees them, so the witches are obliged to set things straight. The witches are symbols of "an eye-for-an-eye" type of fate and destiny in which one receives what he/she gives out. As for Macbeth, the witches tell him that he will become Thane of Cawdor and King of Scotland, but they never tell him how this will happen, nor do they suggest that he murder the king to take his title. Macbeth, with the help of his wife, does this all on his own. Later in the play, Hecate chides the witches because they have given Macbeth too much information, and she tells them that they need to fix what they have started. So the witches are not evil themselves, they are simply the catalyst that brings out the evil in Macbeth.
It is important to note that in Shakespeare's time, there was quite a furor over witches and witch-hunting. We, in the US, know something of this story from the events surrounding the Puritans of Salem and their famous witch trials. So, for the average God-fearing citizen, alive when Macbeth was written, witches were considered a thing to be feared, and there was certainly paranoia about having a witch curse you with black magic.
The question of evil is a major theme in the play Macbeth, and could be considered, like Beauty, to be in the eye of the beholder. The witches cast spells:
Round about the cauldron go;
In the poison'd entrails throw...
Boil thou first i' the charmed pot.
They call their familiars (animal assistants) in the opening scene -- "I come, Graymalkin;" "Paddock calls." And both the cauldron of magical goo and the animal familiars are strong signifiers of witches who perform evil deeds. But does this prove that they are evil? It is hard to say.
It might be true that the witches, by a preponderance of evidence, are evil, but the question that concerns the play more would still remain: Are the witches and their charms responsible for Macbeth's behaviour, or is Macbeth a free agent, acting with his own free will? This, more than whether the witches themselves are evil, is the question that seems to permeate the play.
The weird sisters are evil in nature,as well as in their designs.Since the play begins,Shakespeare points this out.They play upon Macbeth's desires n ambitions,and that's why they decide to meet him "upon the heath".
The incident of the chestnut eating woman is significant,as it shows the witches in an evil light,seeking revenge.The way the first witch says that she"ll render"him dry as hay"and he shall live like a cursed man for 81 weeks.He would "dwindle,peak and pine".
Perhaps the biggest example of their evilness is
"Fair is foul,foul is fair
Hover through the fog and filthy air"
These lines also form the theme of Macbeth in a nutshell.