Why is act three scene four the turning point?
If you see Act III, scene four as the turning point, I'd say it is the turning point for three reasons.
First, and most simply, the plot: Banquo is dead.
Second, on the psychic/spiritual level, Banquo's ghost returns, showing that Macbeth's actions will be protested by the very nature of the universe. This should be a sign that he is not to be king, but he pushes on.
Third, on the psychological level, Lady Macbeth must now take more control of the situation, precisely because her husband is losing control.
Those three things turn the play at this point.