How is Macbeth a weak man who brings suffering upon his own head?
Macbeth is a weak man because he is easily tempted and convinced by forces outside his own thinking and moral code. Macbeth's trouble starts with his own internal desire to possess power, however, he seems to be able to keep his secret ambition just that a secret. He is a noble and loyal servant of King Duncan, who serves him well on the battlefield, distinguishing himself with a great display of courage, enough that he is rewarded by the King.
However, he is told by the witches that a great future awaits him, but, the witches don't give Macbeth any time lines on their predictions. Macbeth seizes the prophecy of the witches and decides that the time is now. Once one of the elements of the prophecy comes true, Macbeth decides to act to engage the rest of the prophecy by killing Duncan himself, that very night to make way for his own succession to the throne.
Even though he debates with his conscience whether he should kill the king, once he tells Lady Macbeth about the prophecy, she uses ever ounce of her ability to convince her husband that he should take the opportunity and kill the king. She convinces him that it is his duty, if he loves her, to kill the king.
Macbeth decides to kill the king, listening to his wife, combined with the witches prophecy, he is crowned king. After his coronation that is when he begins to fall apart. He sees enemies in every corner, believing that he is threatened by everyone, he develops a strong degree of paranoia. This sets him on a course of unnecessary murder.
He kills Banquo and tries to kill Fleance, his murderers don't kill Banquo's son but Macbeth is haunted by the ghost of his former friend, brutally murdered to calm his fears.
When Macbeth decides to seek out the witches prophecy for a second time, he sets a course for murder that is unjustified, having Macduff's entire family slaughtered.
Of course, Macbeth's days as king are numbered, his murdering, his madness and his inability to see how he has disrupted the balance of nature by killing King Duncan, bring suffering on him and everyone in Scotland, including Lady Macbeth, who commits suicide after Macbeth has Macduff's family murdered.