Why do some of our elderly have a change to irrational and hurtful behaviors and some do not?

I don't feel so alone since I've joined this forum. It's hard to understand why my mom is behaving in the manner I've been taught is unacceptable. We've always bumped heads with her treating me like a child all through my adulthood. And she would usually get her way no matter how wrong she was. The woman is my hero. In 2015 She was diagnosed with lung cancer (was free of it for 5+yrs before a recent x-ray show signs of a mass. She doesn't want any treatment. )In 2017 had 6in of Colon removed( biopsy revealed malignant). And has lived with the diminishing health. She quit smoking 2019. Just a year before that the doctor said she needed oxygen at home. She was in the last days to go home from a rehab when the pandemic hit. So she went home by herself with me being at her beckon call in mind. I tried my best to uphold her expectations to be there when she asked and do things in time frame she had planned to get done. We bickered often and that would be a reason we never got anything done. I had known of my mom's manipulative ways to get what she wanted or play out like she needed. She played the guilt trip on me most of time. As an adult my mom didn't see my boundaries as for her when speaking to me with respect as she demanded from me. I was raised by much of the way as DO AS YOU ARE TOLD, NOT AS YOU SEE. I would put up with the demands and slanders for no longer than an hour before I it was break time and needed to get away. When her health took a turn I didn't take her habitual behavior as a sign her disease is the cause. And hadn't until recently. I didn't understand how she could actually think I would react to her demands with the way she would deliver them and expect me not to give back in the same way.

About 6 months ago her anxiety heightened and she began OCD behaviors that were not realistic to reach for a goal to be met. Like regulating temperature inside her house. She didn't have central heat and air to be able to easily control temp and humidity as she wanted to do. All possible resolutions she would disagree to with the excuse that she didn't want to learn anything new because she might forget how it work and it would screw up her medicine routine. She had quit watching TV because she said it destructed her from taking her medicine on time. She was on hospice so she had a prescription for TRILEGY taken once a day, Visiril for allergy, anxiety taken as needed, nebulizer Albuterol taken as needed every 4 to 6 hours, and rescue inhaler. The other 2 or 3 we're OTC fiber, melatonin and aspirin. The nebulizer helped her breathe easier she said. So every 4 hrs she was adamant about using it. I would do what she would ask of me and when in motion of doing it she would correct me. She say one thing and mean another. We bickered alot. Even for a short time after she fell and wound up where she is now SNF long term stay. If I make her aware of her behavior she says I'm not on her side and then the guilt really sets. So I just take to her demands lightly until all attempts to please her fail and then I'll announce my exit will be soon. That sorta calms her down so I sit down in a chair and sit with her in silence for sometimes 3 or 4 hours.
Ive also learned to see her pain in front of mine. She has said that it's not me she lashes out about. She mad at life. There's has not been an enlightened moment for her attitude to change some positive. She is at her end of life and she don't like it.

Id like to understand where the drastic change behaviors come from. For what reasons does one act and behave as they have taught their youth not too and is unacceptable.