One of the most fascinating aspects of Macbeth is that, though Macbeth's future is presented as if fate were guiding it, there's also the suggestion that Macbeth's poor decisions influence and bring about his downfall. For instance, consider Macbeth's behavior after he gains the crown. Though it would appear that Macbeth is initially liked by most of his followers (or, at the very least, he's not disliked), Macbeth becomes increasingly paranoid and embarks on a murderous rampage to quell his fears, first killing Banquo, and then murdering Macduff's family. By doing so, Macbeth rightly earns a reputation as a tyrant, and his formerly loyal followers begin to plot revenge. By extension, we can see that Macbeth's paranoid and tyrannical behavior (his poor decision making, in other words) directly leads to the rebellion of his noblemen and his eventual demise. In that case, even if fate is at work in the play, it's also clear that Macbeth's poor decisions play a huge role in bringing about his downfall.

How is Macbeth doomed by his choices?

Macbeth is doomed by his choices because he kills innocent and righteous people in order to satisfy his own ambition and pride.  Immediately after he kills Duncan, he regrets it, and he fears that he has ruined his soul.  He panics because, when one of the chamberlains awoke and said, "'God bless us,'" Macbeth was unable to say the word "Amen."  He says that he "had most need of blessing" and yet he could not utter the word; this is a sign that he has lost the ability to be blessed.

Further, he chooses to arrange for the murder of Banquo and his son, Fleance, because of the Weird Sisters' statement that Banquo will father kings.  While he awaits news that it has taken place, he tells Lady Macbeth that his "mind is full of scorpions," and he expresses the idea that it is worthless to have gotten what they wanted when they do not feel happy.  He is guilt-ridden and wracked by paranoia and fear, and he cannot even enjoy the position that he killed Duncan to get.  One bad choice leads to a worse one and an even worse one, and so on.

Finally, by the time he orders the killings of Macduff's wife and children out of pure cruelty and malice, it is clear that he has become a monster.  The fact that these murders take place on stage, in front of the audience, emphasizes just how completely depraved Macbeth has become.  In the end, as Malcolm's army marches on the castle, he says, "I am sick at heart," and he complains that he has none of the things that one looks to have in one's old age: "honor, love, obedience, troops of friends" (5.3.21, 27).  Instead, people curse him and only pretend to honor him.  His corrupt and terrible choices have doomed him to such a fate.