Why did my advisor criticize my work for the first time at my Master's Thesis defense?
You cannot go after him for asking questions, that's his prerogative. You can choose to work on your PhD with him, or without him. That's what the conversation ought to be about.

Check out Crucial Confrontations:
http://www.frumi.com/images/uploads/CrucialConfrontations.pdf

With a crucial discussions like that, it is always good to give yourself a pep talk and imagine why would a rational being do something like that. Was is really that bad? Were you his first student, and was he nervous? Could he have had some personal issues to deal with? Perhaps he feels embarrassed for his behavior? 

That will help you develop compassion and temper your anger (regardless how justifiable it might be). The goal is really to channel your emotions.

Then you are ready to develop a focus for your discussion. You can pose it as the discussion about your concerns about doing PhD with that advisor. Let's face it, you were not going to do PhD with him, there wouldn't be need for the conversation. Ask questions, at this point.

For example, I was surprised with the level of question received during the defense. I was left doubting the quality of my work, and now I am concerned about proceeding with PhD because my work is not as good as I thought it would be. What is your take? That should be enough to start the conversation without accusations. 

Keep in mind, they are supposed to ask you hard questions at the defense, it is an exam. It might even be a compliment - he wouldn't have asked you these questions, if he didn't think you can handle them.  My advsor holds practice defenses. The labmates and the adviser ask hard questions then. For my defense practice, I was answering questions for an hour! If it wasn't for the practice, these questions would have come at the defense time.