Quite simply, Macbeth's downward spiral is the result of a combination of great ambition, crippling insecurity, and gullibility.
Macbeth is a loyal soldier of Duncan and of Scotland in the beginning of the play. He fights with bravery and great enthusiasm, slaughtering traitors and bringing justice to those who have done wrong to Scotland. But, his ambitions are great - he desires power; power such as being King of Scotland. When Duncan proclaims his own son the Prince of Cumberland, Macbeth is immediately disappointed and frustrated, even considering murdering Duncan as a way to achieve the throne. Here is our indication that Macbeth wants desperately to be king. His ambitions to wield that power is certainly evident.
Secondly, however, he is insecure. When he tells Lady Macbeth of the Witches' prophesies, that he will be Thane of Cawdor and eventually king, she immediately pushes him to action. Even though he prefers to take a calm and carefully planned approach, he buckles under her pressure when she challenges his manhood by calling him a coward. Rather than believe in himself, he allows his insecurity to get the better of him and, thus, rushes into action at the command of his wife.
Lastly, he is certainly gullible. Rather than question the witches as Banquo has (who believes that they possibly ate a bad "root" and are seeing things...), Macbeth believes them with vigor and immediately tells his wife about their prophesies. Even when things become very tense and he is afraid for his own safety & security, Macbeth turns to the witches for guidance. Why? Probably because they effectively predicted that he would become the Thane of Cawdor. That seemed to lock in his mind their powers and honesty. Unfortunately, the witches have cruelty in mind and are bent on ruining Macbeth, seemingly for sport.
The downward spiral consists of him starting out as a warrior who is brave and victorious in battle and who is loyal to Duncan. It starts downward with him wanting to be king, then killing Duncan. It gets worse when he has Banquo killed (and tries to kill Fleance) and hits its low when he has Macduff's family killed.
There is a lot of debate over who is at fault.
Of course, the witches' predictions set Macbeth on the path by making him think he can be king. And his wife helps him along by pushing him initially, to kill Duncan. But many critics argue that Macbeth himself is the main cause of the downward spiral because he is too power-hungry.