Do the witches *make* Macbeth evil? They don't have that kind of power. They can already work with what is already there. Having said that, they do seem to have awaken something within him. They plant the seeds that he allows to grow into pure evil.
Had they not told him anything about his future, he may have gone along happily fighting on Duncan's sides and, just as happily, being rewarded for his acts. The witches are masters at sowing seeds that humans use to destroy themselves.
Macbeth knows that what the witches tell him "cannot be ill", but also "cannot be good." Unlike Banquo, who immediately suspects the witches motive (he says sometimes the instruments of darkness tell small truths to gain the soul), Macbeth seems willing to suspend his misgivings, even to the point where he almost immediately pictures himself killing his king. Yes, he wrestles with the deed, and yes, he does take some convincing from the mistress of manipulation before he actually decides to go through with the murder, but the fact that he can't decide is not as telling as the fact that he almost immediately entertains the notion of murder. Were this potential not already within him, his response would be more like Banquo's. Macbeth is human, but his humanity is weak for he is all too willing to yield to his own dark imagination.
The witches definitely contribute to Macbeth's choices; however, once Lady Macbeth learn of the witches' prophesies, it is she who convinces Macbeth to go through with the murders: "Art thou afeard/To be the same in thine own act and valour/As throu art in desire? Wouldst thou have that/Which throu esteem'st the ornament of life,/And live a coward in thine own esteem,/Letting 'I dare not' wait upon 'I would',/Like the poor cat i'th'adage?" (I.vii.39-44)
Macbeth seriously contemplates the thought of murdering the king, and he only agrees to go through with it after his wife insults his manhood through manipulation: "I have given suck, and know/How tender 'tis to love the babe that milks me./I would, while it was smiling in my face,/Have plucked my nipple from his boneless gums/And dashed the brains out, had I so sworn/As you have done to this." (I.vii.54-57)
Lady Macbeth does suffer for the actions of she and her husband. Her subconscious taunts her through her dreams, which does give her character sympathetic and redeemable qualities.
It does not seem likely that Macbeth would have even considered murdering Duncan and Banquo if the witches had not intervened.