Macbeth is a weak man, overwhelmed by ambition and drive by forces he barely attempts to repel. Show evidence of this in Shakespeare's Macbeth.

I wonder though if Macbeth remains weak...or if indeed he
was as weak as perhaps some readings like to presume.  Towards the end of the play, when
Lady Macbeth is going mad, the audience sees a stronger and more determined Macbeth.
Granted he does feel invincible because of what the witches have told him.  But he shows
little regard for his wife during the last scenes of her
life. 

Another point to consider is that whilst he was
perhaps encouraged by his wife to commit the crimes initially, I am not so sure that she
is so powerful as to completely influence his judgement.  Admittedly until a recent
viewing of the current RSC production of the play I was totally akin to the idea that
Lady Macbeth was the stronger of the two.  Yet actually Macbeth always had the strength
in him, otherwise he wouldn't have dared to commit the crimes that his wife suggested. 
Did she help him? Yes.  Did she influence him?  Yes.  But who is responsible? 
Him. 

What constitutes his weakness is his failure to
listen to his conscience/divine doctrine.  He is driven by a lust for power, glory and
status and will allow no one to prevent him from achieving his goals.  His state of mind
is thus ambitious and determined.  These traits are not those of the weak minded, rather
they are traits of those who have aspirations.  His weakness lay in his inability to
listen to reason and to let considerably evil notions to influence his judgement. 
Remember though that everything that is considered to be for the sacred honour of King
Duncan at the opening of the play - when Macbeth is praised for the slaughter of
Macdonwald!  Yet ironically at the end he is cursed for his brutal behaviour when it is
against the divine right of Kings.  Perhaps then his weakness is that of a weakness of
the spirit ... just like Eve in the Garden of Eden and the fall of mankind.  Perhaps
Shakespeare is pondering on what a man will do for power?  A clear definition of
weakness here is paramount ... because on Godly terms Macbeth was weak, totally
corrupted by the forces of evil for his own self gratification.  Yet by the acts of
violence one could argue he is not weak at all physcially, but his insecurity and lack
of self esteem gradually show his weakened state of mind as one by one the characters
around him are killed off, to leave a King holding on to his power right until the end
when it is taken from him.