Ambition is Macbeth's tragic flaw. Both Macbeth and Lady Macbeth are guilty of this. It is this 'vaulting ambition' (I.7.26) that results in their moral, spiritual and physical demise.
Macbeth: AS & A2
Both Macbeth and Lady Macbeth are tormented by guilt, as each slowly goes mad, plagued by the guilt of their actions.
In Renaissance times, the Great Chain of Being was believed to hold the world together. By killing Duncan, Macbeth disrupts the natural order in the world.
Duncan and his son Malcolm symbolise all that is right and good. When order is restored at the end of the play, grace is also resurrected.
Lady Macbeth recognises that how she and her husband appear is paramount. They must 'look like the innocent flower / but be the serpent under't' (I.5.63–4).