Why is Act Three, Scene Two the turning point of Macbeth?

One can make an argument for many scenes being turning
points in Shakespeare's Macbeth:  Lady Macbeth's shaming Macbeth
into killing Duncan; Macbeth's killing of the grooms (a veering from his wife's plan
which suggests his guilt to at least Macduff); Malcom's acceptance of Macduff as an
ally, etc.  Act 3:2 is not a scene that stands out as a turning
point. 

If you must answer a prompt emphasizing Act 3:2 as
a turning point, however, the scene does demonstrate Macbeth's emergence as the person
in charge, as the man, so to speak, who wears the pants in the family.  Instead of
relying on his wife to strategize and make plans, Macbeth has taken over the planning,
as evident in this scene.  Ideas of how to proceed are now his own.  He makes the plans
and carries them out without his wife's participation or consent.  Act 3:2 reveals his
plans to have Banquo assassinated.  His wife is totally unaware of these plans.  He has
reversed the roles the two played in Act 1.   

Of course,
this is to Macbeth's detriment.  He might have gotten away with killing Duncan if he had
stuck to his wife's plan.  Lady Macbeth is the planner of the family.  Macbeth makes
mistake after mistake once he takes over.  Instead of relying on his wife, he decides
what to do on his own.  Worse, he also uses the witches as advisers.  His poor decision
making dooms him. 

This is made concrete in Act 3:2.  His
wife is worried about their security.  Macbeth tells her not to worry.  He has taken
care of the situation.  He has ordered the assassination of Banquo.  Of course, Banquo's
assassination is just another coincidence that eventually leads Macbeth's lords to turn
against him.  And Fleance's escape ensures that Macbeth will leave no legacy of kings to
rule Scotland.