This is a really difficult question to answer, mostly because Shakespeare leaves the notion of fate in Macbeth as fairly ambiguous. The answer will change depending on how you interpret Shakespeare's work, so you should view the following answer as my particular interpretation. Other scholars might differ in their views, and they would be justified in doing so.
Here's my take on the issue: there is, of course, the Weird Sisters' initial prophecy that Macbeth will be king, and so there seems to be a certain amount of fate at work, which would suggest that Macbeth is indeed a puppet of larger, universal forces and is not fully in control of his own actions. However, while Shakespeare begins the play with a cryptically prophetic statement on the gloomy moors, he never fully commits to the idea that Macbeth is a mere puppet being used by fate. No matter what fate might "plan" for us, Shakespeare suggests, we always have the ability to choose.
For instance, even though Macbeth is prophesied to become king, the Weird Sisters never explicitly say that he is fated to kill King Duncan and usurp the throne; that's merely a result of Macbeth and Lady Macbeth's interpretation of the prophesy. Therefore, it's possible that Macbeth could have chosen to remain a loyal and honest man, and that he could have become king by honorable and legal means later in life. Since the Weird Sisters never specifically describe the manner by which Macbeth will become king, his method for assuming the power of the crown seems to be a force that he controls. However, Macbeth rushes to fulfill the prophesy by murdering Duncan in his sleep, and so he chooses the quick and violent path. The fact that Macbeth makes this choice with his own free will is the true tragedy of the play, as it proves that this formerly honorable man is capable of using his free will to perform great evil. Thus, the quick answer to this question is that, though fate could be at work, Macbeth is in control of his decision to murder Duncan, a fact that makes this act all the more hideous.