It would seem that Shakespeare wanted Macbeth to be a sympathetic character and not just another villain like his Richard III. Shakespeare takes pains to make it appear that Macbeth was powerless to avoid doing exactly what he did and ending up exactly the way he ended up. In fact, Macbeth seems to be fighting against fate or destiny more than against any human enemy. For example, he has been told by the Weird Sisters that Banquo will father a whole line of kings. He tries to forestall fate itself by having Banquo killed--but Fleance escapes.
In Act 3, Scene 1, when he is thinking of having Banquo killed, he finishes his sololiquy with the lines:
Rather than so, come fate into the list,
And champion me to th' utterance.
He is challenging Fate itself to combat. This courageous attitude makes him a somewhat sympathetic character.
In Act 4, Scene 1, he is told by the First Apparition
Beware the Thane of Fife!
But after being assured by the Second Apparition that no man of woman born can harm him, Macbeth says:
Then live, Macduff; what need I fear of thee?
But yet I'll make assurance double sure
And take a bond of fate.
To demand a bond of Fate is a wonderful concept. Once again he is opposing Fate.
Macbeth believes in fate or destiny and at the same time distrusts it. If it is his destiny to be king until Birnam Wood moves to Dunsinane, etc., then he will trust Fate; but if it is Banquo's destiny to father a whole dynasty of kings, then he will oppose Fate. In the end Fate proves invincible, and Macbeth is its hapless victim.