Macbeth's most obvious character flaw is his ambition. This drives him to commit the murder in the first place. He admits as much in his soliloquy at the end in Act I, Scene 7:
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other—
Macbeth's ambition is largely responsible for his destruction, as well as the demise of most of the people around him. Another character flaw of Macbeth's is his willingness to be swayed by his wife and the witches. In the soliloquy quoted above, Macbeth has essentially decided not to go through with the murder of Duncan, but his wife's goading spurs him toward the task. By the end of the play, he has placed so much stock in the witches' prophecies that he genuinely believes no man "of woman born" can destroy him. He does not pause until the bitter end to consider that the witches are essentially toying with him.
Over the course of the play, we see Macbeth's unnatural rise to power has corrupted him. He is a bloody tyrant, and has Banquo and Macduff's family murdered to solidify his position as monarch. Macbeth's violence is emphasized early in the play — he "unseam'd" the rebel Macdonwald "from the nave to the chaps" in battle. This act, evidence of his valor and loyalty to Duncan early in the play, grimly foreshadows Macbeth's violence later in the play.
In summary, three of Macbeth's many character flaws are his ambition, credulity, and capacity for violence.