In Shakespeare's Macbeth, there are a number of causes for Macbeth's ultimate destruction.
The most noted ones are his tragic flaw of "vaulting ambition"—Macbeth says he just cannot say "no" because his ambition to be king overshadows everything else. Another is the "false sense of security" brought on by the witches' prophecies, which allow him to believe he cannot be beaten.
There are, also, other factors. A prominent one is his failure to remain steadfast within his own value system. I know that he blames this on his tragic flaw (without saying those words), but if a man can face raging soldiers and the slashing of swords, etc., on a battlefield between Scottish and Irish fighters (known for their frightening battle cries), how can this man not be strong enough to stick to his guns with his wife? This is one aspect of Macbeth's character that troubles me—how can you be so noble and give in because of the venomous insults from your wife?
My hands are of your color, but I shame
To wear a heart so white. (II.ii.80-81)
Perhaps if Macbeth had been portrayed as a man obsessed with his wife (like Othello), I could understand how easily he turns away from his conscience. The fact that his personal character finds his actions unnatural is further demonstrated with how conscience-stricken he is just before, and immediately after, Duncan's murder—it amazes me that he could fall so far.
Macbeth also forgets to use his common sense. Every member of Shakespeare's Elizabethan audiences would know better than to place their [complete] trust in the witches. These people lived with the certainty that witches lived among them, and that their only purpose on earth was to trick mortals into relinquishing their immortal souls. Even had Macbeth been ambitious enough to listen to them, he seems savvy enough as a warrior that he might have exercised caution in placing his entire trust in them. It is only too late that he realizes he has been tricked.
And be these juggling fiends no more believed,
That palter with us in a double sense,
That keep the word of promise to our ear… (V.viii. 23-25)
On the other hand, perhaps this shows us more clearly how human Macbeth is, and explains why he loses sight of things most men in his "military capacity" might not overlook (e.g., Banquo). Macbeth shows us just how fragile he is, and why Shakespeare has created, to some extent, a sympathetic protagonist. It is not unusual for people to have the best of everything and put it all on the line to have more. Macbeth has proven himself to his peers, has been rewarded (and is loved) by his King, and his future looks bright, yet the temptation for "the prize" is too strong and he is lured away from all of this. Being human, of course, is why his ambition drives him so easily, why he gives in to his wife's nagging, and why he places trusts in the witches when he should not: his is as imperfect as the rest of us. However, his poor choices bring about his eventual death.