says, "Of all men else I have avoided thee/But get thee back; my soul is too much charged/with blood of thine already." Once Macbeth understands that the circumstances of Macduff's birth exclude him from another of the Weird Sisters' "protective" prophecies, he understands that Macduff has every reason to kill him since Macbeth arranged for the murder of Macduff's family.

Macbeth is his own worst enemy.  A really good example of this is after he kills King Duncan, and instead of sticking to the plan, he kills the guards.  “Oh yet I do repent me of my fury/that I did kill them!” (Act 2/sc 3, line 107)  Macbeth panics, thinking that leaving the guards alive will somehow come back to haunt him.  In reality, this is the moment when his friends first start to suspect him.  Eventually, their suspicions are confirmed and they rebel against him.

Another example of Macbeth’s actions leading to his downfall is the decision he makes to kill Banquo.  Banquo is his best friend and the only person to have witnessed the predictions made by the witches.  Once Macbeth and Lady Macbeth carry out their plan, Macbeth begins to think that Banquo’s honorable nature will prevent his friend from supporting him as king.  “Our fears in Banquo stick deep/And in his royalty of nature reigns that which would be feared.” (Act 3, Sc. 1, lines 49-51)  At the end of this important soliloquy, he has decided to kill his friend. “Rather than so, come, fate, into the list, and champion me to th’ utterance” (lines 71-72).  The reader can perhaps understand his ambition leading him to kill Duncan, but the murder of his best friend is much harder to justify.  His inability to try to protect himself and his crown at any cost is what leads to his downfall in the end.

The question of Macbeth's responsibility for his own downfall is a significant one in the play. Macbeth himself often seems unsure of how much action he needs to take to make his ambitious dreams a reality. He initially hopes the witches' prophecy of his impending coronation will come true without having to do anything about it, but when it appears otherwise, he more seriously contemplates regicide as a means of helping the prophecy along.

While he wavers in his resolve to kill Duncan, Lady Macbeth's influence steels it, especially when she starts insulting his sense of manhood. At the end of act 1, scene 7, Macbeth makes clear that he is willing to do whatever it takes to be king, a statement which solidifies the idea that he knows full well what he is doing, and what he is risking physically and morally in assassinating a rightful king:

I am settled, and bend up
Each corporeal agent to this terrible feat.
Away, and mock the time with fairest show;
False face must hide what the false heart doth know.

In other words, Macbeth is wholly committed to taking the bloody path to power and luring King Duncan into his home with a "false" friendly front. The witches and Lady Macbeth only needed to give him a small push to bring that conviction about. Macbeth's words in this scene ring with a sense of responsibility and purpose, and they aid in making Macbeth an active agent of his own demise rather than merely a dupe of the supernatural or fate.

Macbeth always believed that the witches' prophecies were not just true, but would offer him protection. So when he realizes that that's not the case, it's a rude awakening.

The witches aren't responsible for Macbeth's downfall; Macbeth is. And we see the deadly consequences of his actions when Birnam Wood finally moves against him.

According to one of the apparitions conjured by the Weird Sisters, Macbeth would never be vanquished until Great Birnam Wood moved against Dunsinane Hill. Not unreasonably, Macbeth thought he was safe. After all, whoever heard of a forest moving?

But in the event, the prophecy came true because the invading rebel army has moved against Dunsinane wearing camouflage taken from the branches of the trees in Birnam Wood. Macbeth realizes what is happening and, in a soliloquy shows that he is responsible for his own downfall:

Ring the alarum-bell!—Blow, wind! Come, wrack!
At least we’ll die with harness on our back. (V.v.50–51)

Birman Wood really is moving against Dunsinane Hill, though not in the way that Macbeth envisaged. And it's all because Macbeth murdered Duncan. No one else is responsible for his downfall; not the witches, not Lady Macbeth, not anyone else.

Now that he's finally realized that the witches' prophecies offer him no protection, Macbeth resolves to die if necessary in battle with his armor on.

The witches never tell Macbeth that he will have to murder Duncan in order to be king. But upon hearing their premonition that he will be king, this thought enters his mind. He is terrified at the idea, but he has the thought nonetheless: 

My thought, whose murder yet is but fantastical, 
Shakes so my single state of man that function 
Is smothered in surmise. (I.iii.150-52) 

Later in this scene, Macbeth even says that the may become king without doing anything at all. So, he does suppose that he won't have to murder. 

In the next scene, Duncan alludes to the possibility that the Thane of Cumberland, Malcolm, has a chance to become the next king. In an aside, Macbeth sees this as an obstacle: 

The Prince of Cumberland! That is a step
On which I must fall down, or else o'erleap
For in my way it lies. Stars, hide your fires, 
Let not light see my black and deep desires: (I.iv.55-58) 

Macbeth has these two thoughts before he converses with his wife. There is always a debate and discussion as to how much Lady Macbeth motivates Macbeth to murder Duncan. Some critics say she is the driving force behind Macbeth's transformation from loyal thane to murderous tyrant. But he has these murderous thoughts before he speaks to her about the witches. 

Also, later in the play, it is his decision to murder Banquo and Fleance. He even refrains from telling her at the time. Lady Macbeth asks him what he is up to and he replies, "Be innocent of the knowledge, dearest chuck / Till thou applaud the deed." (III.ii.50-51) Lady Macbeth does motivate and encourage Macbeth throughout his downfall, but the initial thoughts are his. And when he becomes king, he has this additional idea of murdering Banquo and Fleance. Lady Macbeth has nothing to do with this decision. But Macbeth knows she will certainly "applaud the deed."