Perhaps the first turning point in Shakespeare's Macbeth is also the most important: Macbeth and Banquo meet the three witches in Act 1:3, and the witches hail Macbeth as "thane of Cawdor" and "king." This serves as the catalyst (gets the action moving, sets the direction for the action, creates the complication) in the play.
Until the witches ignite Macbeth's ambition (his reaction as revealed by Banquo in Act 1:3.52 suggests he has considered the possiblity of being king before and knows that it will take assassinating Duncan to achieve it), he appears to be a loyal subject of King Duncan and is a highly respected general. He's already achieved greatness, and is loved and honored by the king. The rebellion occuring in the play's opening scenes has been squashed and Scotland is again at peace.
That all changes, or "turns," if you will, when Macbeth, thanks to the little push by the witches, decides he needs to be king sooner rather than later. Scotland will not be peaceful for long.
Another key turning point occurs after Macbeth has second thoughts about committing a heinous act like killing a king, and he informs his wife that he's changed his mind in Act 1:7.31-35. Lady Macbeth spends the rest of scene seven convincing Macbeth to go ahead with the assassination. After his wife's elaborate arguments, Macbeth pronounces that he "is settled" (Act 1:7.79), and they move ahead with the assassination plans.
A few other turning points in the play include:
- the actual assassination
- Fleance esscaping from his would-be-murderers, which means Macbeth can't create his own dynasty
- the ghost of Banquo appearing at the feast, which leads to Macbeth's public breakdown
- the equivocations/deceptions of the witches being revealed (the woods do move and Macduff wasn't born of woman).
One key turning point is one we only get a whiff of--Lady Macbeth's breakdown. We see the results, but few details are revealed about the process she undergoes. We only know that Macbeth stops consulting with her and relying on her to make plans. Unfortunately for Macbeth, that, too, is a turning point. She's the planner of the family.