The witches did nothing but stir up what was already there. Look at how quickly Macbeth and Lady Macbeth latched onto those prophecies. If Macbeth had been like Banquo, and had been suspicious of them, it would have gone no further.
Whether or not they report whats' "destined to be" depends on whether or not you believe the witches have that power in the play. I tend to think they do because of their prediction that "none of woman born shall harm Macbeth" and the idea that Birnham Wood will come Dunsiname. Both of these come true, and they are not events based on the manipulation of Macbeth's ambition. So, no, I do believe the witches caused what happened to Macbeth.
They are the instigators. They do not cause Macbeth to do the things he does, but they help to put the ideas in his mind. There is not destiny involved. It is Macbeth who is actively choosing to do what he does. The witches suggest it to him, but he is the one who chooses to act.
yes, i think they are partially to blame for macbeths action as they create a kind of paradox, if they had not have told him of what is to come he would not have strived to acheive it therefore they created the chain of events by 'foretelling' it.
however i do think that lady macbeth is also responsble, possibly more responsible, sinse she manipulates macbeth to kill duncan by insulting his masculinity and emotional blackmailing him into following her command.yes, i think she is more responsible, she constantly pushed him toward this convincing him it was his goal to be king and his desire, for example: 'to be the same in thine own act and valour, as thou art in desire' he was a brave soldier and she convinced him he had to be brave in his attempts to acheive royalty.
Although the witches are responsible for tempting Macbeth, they cannot be held responsible for his actions. The witches are unable to control fate, which is displayed in Act 1, Scene 3, where the witch is able torture the sailor, but not ultimately kill him. Therefore, the witches had no control over Macbeth's fate. They simply influenced it by playing on Macbeth's weaknesses, because according to his surprised reaction, it is not the first time he has considered becoming king (and perhaps not even the first time he has considered murdering Duncan).