Shakespeare gives numerous clues throughout the text as to which character truly is the leader in the conspiracy to kill King Duncan. Even though the witches plant the seed of ambition in Macbeth's head in Scene 1, he would never have the guts to do something as black as murder the reigning king without pressure from Lady Macbeth. She, as his wife, knows him best, and says of him:
"Glamis thou art, and Cawdor; and shalt be What thou art promised: yet do I fear thy nature; It is too full o' the milk of human kindness To catch the nearest way: thou wouldst be great; Art not without ambition, but without The illness should attend it: what thou wouldst highly, That wouldst thou holily; wouldst not play false..." (Act 1 Scene 5).
She states that though Macbeth may have ambition and potential for greatness, he is too good to act in any false way to attain his goals. This is where she decides to become the influence he needs to make quick work to fulfill the prophecy. She calls upon evil spirits to fill her with evil power. Read the whole scene to get the full effect.
"Come, you spirits That tend on mortal thoughts, unsex me here, And fill me from the crown to the toe top-full Of direst cruelty!" (Act 1, Scene 5).
Later in the play, her dominant role is reinforced when Macbeth has second thoughts about the murder (Act 1, Scene 7). She tells him he cannot call himself a man unless he does the deed. Throughout the whole ordeal, Macbeth is clearly struggling with fear and guilt, while his wife is undaunted. After the murder is complete, Macbeth is the first to hear voices and suffer fear of being discovered--Lady Macbeth again becomes the voice of rationality,
I'll go no more: I am afraid to think what I have done; Look on't again I dare not.
Infirm of purpose! Give me the daggers: the sleeping and the dead Are but as pictures: 'tis the eye of childhood That fears a painted devil. If he do bleed, I'll gild the faces of the grooms withal; For it must seem their guilt" (Act 2 Scene 2).
In all of these scenes, Lady Macbeth seems to be the voice of darkness and temptation for Macbeth. Yes, he does commit the murders of Duncan and his servants, but after Lady Macbeth goads him to go through with it. After all of this, the spirits of darkness that she called upon at first seem to have abandoned her to her guilt and fear, and she dies having lost her mind.