You are in a tough spot. That future mother-in-law is going to be a perpetual source of grief to everyone around her. Your fiancee and his mother are "a package deal" - if you take one, you get both. MIL has had years and years to brainwash and train her son into catering to her evil moods. He's going to always be trying to please her and trying to make you please her. Problem is: she can't be satisfied. So being around her will cause your fiancee and you endless frustration.

This woman sees you as a rival. She resents the affection her son has for you. She figures you're taking her son away from her. A person like that is a trouble maker . . . and she's unlikely to ever change. That's why I say that you're in a tough spot.

You have absolutely no obligation to like this woman, but you are obligated to try and be courteous around her. So try to always say, "Hi." - went you enter into her presence, regardless of what mood she's in. Then, at least, you've done the correct thing. How she responds is on her. It's not your responsibility to make this lady happy, which is most likely impossible.

Don't get between mother and son. That's a no-win situation. Your fiancee is a grown man. He can physically handle his mother. It's not your place to try and stop her from hitting him. You do have a right to walk out of a room where that kind of nonsense is going on, which I recommend you do. You can even walk out of the house. This woman likes an audience. Don't be in the audience.

You need to make an intense study of how to set and maintain boundaries. I highly recommend you find a therapist to help you with that, or at least start reading and watching videos on that subject.

Do not stay around this woman, if she starts to physically assault you. Leave! Walk away. Tell your fiancee that you will not remain in that woman's presence when she becomes physically threatening. That's your right and your responsibility. If he argues with you, say that this is not negotiable. I'm holding out the hope that your fiancee will learn from your good example.