Where does the force of evil lie in Macbeth?

The discussion in class today seemed to lead to a split indecision of who was and who was not evil and whether or not the idea of anyone being evil was even present in Macbeth. Many were able to confidently argue that the three witches were evil because they were manipulative and caused Macbeth to kill. On the other hand, just as many people were able to argue that the three witches were not to blame for Macbeths killings, but rather that they simply released information about the future and Macbeth acted irrationally. Personally, my idea of the story would somewhere in the middle. I don't think that the witches intended for Macbeth to kill, although they were witches and could see into the future so they must have known how Macbeth would react. Those relaying messages can not be held accountable for the reactions they cause, they may be somewhat to blame, but in the end, they are not commiting murder. Macbeth holds more of the blame because of how he intrepreted the information and how he decided to react to it. There again, I would not completely blame Macbeth either, because it seemed like Lady Macbeth played a large role as well. Macbeth was undecided as to what he should do, the thought of murdering to become king had crossed his mind, but he rationalized away from that idea. Only until Lady Macbeth talked him into it. She was the manipulative character in Macbeth because she knew how to talk her husband into doing what she wanted him to do, and she knew that she had the power to do so. In my opinion, Macbeth is just a very weak and easily manipulated character. The witches are just a part of the story, but at no point in the story was the blood on their hands so they can not be to blame. Lady Macbeth was extremely manipulative and pushed Macbeth into doing what he did, and she did end up with blood on her hands, so she may be just as much to blame as Macbeth. Macbeth actually commited the murders, but would he have done so with out the encouragement of Lady Macbeth? I'm not sure if he would have or not, but I would lean towards probably not. For this reason, if there really is the element of evil in Macbeth, I would say that it lies with in the character of Lady Macbeth.