The flaw in Macbeth's character that brings about his own destruction, both physical and spiritual, is that of rampant ambition. He desires political power above all else.
When he is tempted by the witches with the prophecy that he will become King of Scotland, Macbeth's latent ambition for power is evoked. He resists acting upon his ambition for a time. When he first hears the prophecy in Act I, Scene iii, Macbeth is tempted, but his reaction soon turns to one of passive acceptance: "If chance will have me King, why, / chance may crown me, / Without my stir." He does not maintain this philosophy for very long.
Influenced by Lady Macbeth's urging (and very effective psychological manipulation) in Act II, Macbeth soon murders King Duncan, as well as his grooms. Macbeth covers up the crime by blaming Duncan's sons, Malcom and Donalbain, and then goes to Scone to be crowned.
The prophecy has come true; he is King of Scotland. However, by murdering Duncan, Macbeth has fallen from grace. He knows that he has given his soul, his "eternal jewel," to the Devil himself. He soon falls from power, as well, through his continuing murderous acts.
Before the play reaches its tragic conclusion, Macbeth murders Banquo and Macduff's entire household. He becomes a monster through unfettered ambition: He equates being great with being powerful and views the crown of Scotland as "the ornament of life." All that remains for Macbeth is his own death.
I would add that Macbeth also was easily influenced, he was indecisive, weak of character, not sure of himself. Clearly, it is Lady Macbeth's prodding that convinces Macbeth to go through with the murder of King Duncan.
After hearing the witches prophecy, he struggles with his conscience and decides that he will not kill the king. However, after his manipulative wife gets through with him, he kills Duncan, I think against his own conscience, which immediately leads to the start of his mental breakdown.
He is tormented after that and becomes even more susceptible to suggestion. He is frightened and insecure. He lashes out as his torment increases, which leads to more murders.
Macbeth does not really enjoy his time as king. As king he barely enjoys one celebratory dinner before it is interrupted by the ghost of Banquo. After that, he loses everything in his life that mattered, his friends, his wife, his peace of mind and his ability to sleep and eat.