Could mother's nastiness be caused by her trying to to stop herself from feeling guilty?

Yes, absolutely! It is very insightful and loving of you to see this underlying cause of her behavior rather than letting it cause you to become emotionally upset or disturbed and react. Instead you are responding and that means you are being rational rather than emotionally driven.

You can't stop your mother's emotions from driving her but you can prevent a runaway train by keeping in mind that she's coping with her disturbing feelings the only way she knows how and the same way she's probably done all her life, being taught that by her own family, it seems, from what you've shared.

When we get in our twilight years, we lose control of so many things! Emotional blackmail seems to be one way many attempt to try to retain some portion of control, whether it is functional or not. Of course, it never is.

My mom tends to do the same thing, albeit in various ways other than outright saying it, and she never did that when I was growing up. Not much, anyway. But I just deal with it the best I can without giving in to it but still trying to alleviate her own feelings of loss of her independence as much as possible while keeping safety in mind. It is a tightrope walk, for sure!

What I've learned is that different parts of the brain are affected in people. Don't you think this behavior change could be as simple as that. I'm only guessing but at a very interesting lecture I attended they were associating certain behaviors with this. eg. Inappropriate and constant sexual statements were from frontal lobe damage, etc.

After my mother fell and had to go to the hospital, they found congestive heart failure. I believe a lack of oxygen in her blood was the cause for her argumentative nature, for the last few years.

My Dad pretty much does the same thing to me. They have poor copeing mechanisms due to their age because they cannot control anything else in their life anymore. My Dad thinks I should feel the same as he does whether it is about politics or my siblings. If he can control my feelings then he can control more and more about his life as I would just do everything to keep him happy. Such as go when he wants to go and only where he wants to go. When they start loosing that grip they no longer know how to cope other than finding ways to get there way or last say so. Hope this helps. I know what you are going thru and it is not easy.

Based on what you have said, I believe you have a good handle on this behavior and what motivates it. Trust your gut. There are a million reasons for the same behavior in a million different people. But here you are LIVING with her, not just seeing that she has the basics. I have noticed in our parents generation(s) - our parents range in age from 78-92 - that burying something they consider shameful rather than just admit it, figure it out and move ahead, is very common.
My husband suspects that his mother, who is the 92 year old, had an affair when he was in high school. She and his dad were married almost 60 years. All the time I knew them, she was attentive to him, took care of him, and he seemed to adore her. He died of congestive heart failure a few years ago in a nursing home where she is now in her final times, with hospice visiting due to a slow growing cancer. She seems to feel very guilty for almost everything and wishes she 'had done more' or had thought differently regarding almost everything. She also has expressed regret frequently about things that are ancient history. This suspicion that my husband has is based on him seeing her kissing a neighbor when he came home early from school. She doesn't know he saw her and they have never talked about it. He never told anybody but me until a few years ago. \
My husband loves his mother, admires his late dad, and constantly tells he what a good mother she was, what a good upbringing she and his dad gave him. He sees no point in making her feel in any way bad for anything she ever did and chooses to only see the good in her. His treatment of others is why I adore him! But often we have talked about how sad it is that his mother won't let herself off the hook, so to speak, for so many things. I am sure she felt if anything weighed on her conscience, she just had to deal with it herself. I think she is 'doing a penance' by living in the same miserable nursing home her husband passed away in. We live in another state and many times have tried to get her to live where we are, so she could be in a nicer place.
In a different way, my own mother I believe has huge guilt, because she has admitted that she 'never bonded with' me when I was a baby and claims my grandmother 'stole' me (ridiculous) because it 'explains' in her mind why she just never did really love me as she did her other children. The awful consequence of her guilt, though, is that she has managed to blame me for the fact that she feels so little for me and she treats me so nastily as compared to my siblings. As if it were my fault that she didn't love me, because she just can admit it in her heart that she would be the kind of mother that just didn't love her daughter. Guilt and remorse is a terrible thing. I have a hard time even having contact with my mother at all; she has gotten much more abusive as she has gotten older. But if I had to make sure she had proper care, I would. I think we need to do the 'right' thing, but we also are not required to serve ourselves up to be abused. Don't take on HER legacy and become guilt filled for your feelings. If you need to talk to a counselor just to find more peace and clarity, then do that. There is nothing wrong with you. Try to keep your chin up!

Believe what you feel, and trust your instincts. They're right on the money. Flipping the script to have the last word or trying to tear you down are coping mechanisms people like your Mom use to justify their behaviors and live with their conscience. Two wrongs don't make a right, so whenever she goes "episodic" just disengage. Two of my most useful phrases in this kind of situations are "We're not having this conversation" or "Talk to me when you're sober." For your Mom, I'd use the first one.

I think that you've got this pretty well figured out. She is trying to make you the "bad guy" so that she doesn't feel guilty. She's fragile and striking out. Try not to let her push your buttons - she's got a history of that apparently. If you can detach from things so you don't get into arguments, life should be more tolerable. Sometimes just letting things roll off is the best "action" there is.
Take care,
Carol