Is it normal for a dementia patient to know who everyone is but still be hateful and mean to them?

There is NO NORMAL in dementia. NONE. Whether your mother was a Saint or a demon or anything in between, if she has been formally diagnosed as having dementia, it’s all bets off.
Whether your father “tries with her” or doesn’t, her reactions will ALWAYS result from dementia and NOT from what he does or doesn’t do.
I am telling you this without the slightest sense of being “mean” to you. It is a painful and cruel and difficult truth, but once you have accepted it as HER reality, that she is functioning with an incurable brain damage, you will find yourself more free to make the best decisions for her from your own reality based thinking.
Mental illness and dementia can absolutely occur in anyone at the same time, but dementia will always mean that her reactions will be unpredictable. If she was insulting, kind, hostile or pleasant in her past, she will now be expressing emotions in different or similar ways with NO FILTER

You and your poor father need not, and SHOULD NOT, try to justify ANYTHING she says or does. She is at very least, very sick, and you and your dad are vulnerable.

Most of us here have experienced something similar, and learned our own techniques for navigating the assaults, whatever they turn out to be.

WE CARE, and you are NOT ALONE.

Patti, I was just thinking about something. I work in a Memory Care community as a receptionist. We have a woman here with dementia; her husband had to have her placed because one day, she became SO mean towards him that he had no other choice. She's very sweet tempered, in general, except when HE shows up. She turns into a snarling growling little thing and practically attacks him, calling him stupid and useless, etc. No matter what, he takes her out for ice cream or for a walk, with her belittling him the entire time. This behavior is a manifestation of her dementia; they've had a good marriage for like 40 years until the dementia hit her, according to the husband. Just more food for thought.

"She on the other hand, has been hitting him."

I am concerned about your father. If your mother is abusive, one of them needs to be removed from the home. Please do not allow your mother to continue to bring physical/emotional harm to your father.

A family friend had just been diagnosed with dementia. Her husband was making breakfast one morning, he asked a simple question about how she wanted her eggs. She picked up a sharp knife and stabbed him 9 times! She was in a rage and incredibly strong. He had to break the kitchen window to leave the house to get away from her. She was admitted to Memory Care, after a short stay at a Psych unit to find the right meds to calm her rage.

So, you never know. Your Mother is being mean to your Dad. She’s hitting him. Don’t assume it won’t escalate. Your Dad needs protection. I wish you the best as you travel this awful road. Stay strong.

So this is not new behavior for your Mom? You had her briefly committed and said that tweaking medications helped her, but if she is back home it is unlikely she is taking those medications voluntarily.
I can tell you that it is very unlikely that "kidney function" has anything whatsoever to do with that. Failing kidneys often most often result in exhaustion and not a lot of "fight". Urinary tract infection is more likely if this is a sudden change.
We all at a certain age get some "memory issues". But a neuro check is in order. It is sometimes difficult to tell, if things are more chronic, whether you are dealing with mental illness, dementia, or a combination.
If you feel your father is suffering, and if he wishes it, he may need to leave this situation and enter care. In fact, that may be an option for both? But I don't know that they should be together.
I don't see how we could be mean to YOU. None of this is your fault in any way, and there is very little you can do about it; you clearly care and are doing all you can. I am so dreadfully sorry you are going through this.
Aging and depression of loss after loss doesn't do our personalities much good usually. You occasionally will hear of an elder who was once mean and has gone "sweet" but I can tell you that is more rare.
Let us say you were rich as Midas and could afford 24/7 care and monitoring; there STILL may be little you could do about this. Please don't give up your life to it. If your Mom is not safe where she is then APS needs to be told it is not safe for her to be in the home with your father, she needs care, and you need assistance in placing her.
You will have some awful choices ahead. For instance, if you need guardianship that is both costly, and dreadful to do. You may want to consider options by visiting an Elder Law Attorney, and guardianship of the State would mean you don't lose every moment of your entire life going forward worrying about this and addressing it mentally, emotionally and financially (were you to attempt to manage their finances).
The awful thing is I can't see this going any place good. You are going to need help. Is there support of other family there for you? My daughter is far away and works full time, but sometimes just the sound of her voice helps me.

It seems that there are multiple problems going on here.
1 - If her kidneys do not function well, toxins can build up in the blood stream. This can cause confusion and irritability.

2- If she suffered trauma/abuse in her past, she may be reliving the experiences as her dementia progresses. Anti-anxiety medications and lots of reality orientation will be your go to's. Reality orientation is gently reminding her how she is related to everybody and that they care for her. Reminding her that the people around her will not hurt her.

3 - If she has dementia, she will lose the current memories and "walk backwards in loss of memory" - recent events will not "stick", more recent past memories will slowly become lost, and eventually all memories are lost. Dementia also seems to remove the social filter and these folks blurt out whatever they are thinking. The thoughts may not be accurate since there are gaps in memory and their brains are trying so very hard to make sense of everything. Confusion may cause anxiety and/or fear. Those can be expressed in verbal outbursts or violence. Sedatives and anti-anxiety meds may help. Sticking to routines and familiar settings can help lessen the anxiety as well.

4 - Sundowner's Syndrome is a very real phenomena among older folks, especially those with dementia. As the afternoon goes into evening, these folks get confused about surroundings and activity around themselves. The change in light makes shadows or changes the way things look. Dementia with Sundowner's can be treated with more light, especially in the evening and consistency in late afternoon/evening schedule.

Yes, I am an RN and had patients with all the problems you mention. Personally, my gram had slight dementia and Sundowner's. She would get into a thought "do loop" and kept mulling over - and fuming - about a family business disagreement that happened when my mom was a young adult. I kept reminding Gram that we had decided to "forgive" the people she was mad at and reminded her of the last time she saw them and had good conversations. She would be more fearful when she was out of the house in the evening. She had a bout of prolonged confusion when she visited us in Colorado. It ended up being resolved with supplemental oxygen for the duration of the visit. We tried very hard to keep to her usual routine while she was in the unfamiliar surroundings of my home.

Having dementia does not necessarily mean the person will not recognize their loved ones. My 92 y/o mother has moderate dementia and still recognizes everyone..........but she's mean as a snake!! She also treated my dad very badly, especially towards the end of his life, but not as badly as your mom is treating your poor dad. I'm kind of at a loss as to what advice to give you here........just to say that your mom may have a combination of dementia AND mental illness, which is what I believe my mother has. She's always been anxiety ridden with OCD, prone to hysteria her whole life with a myriad of issues. But then, when she started down the dementia road, EVERYTHING was intensified. Her moods worsened, her mouth got even more foul, she got more agitated, more argumentative, more complaining, etc. I guess when you combine dementia with any kind of mental illness you have a recipe for REAL disaster!!!

Call her doctor again right away and see what he says. I can't imagine a kidney problem would cause such a behavioral problem, right? How about a UTI (urinary tract infection)? Was she tested for that? Sometimes, a UTI can cause horrible/extreme behavior in a person!!

Sending you a big HUG and wishing you the best of luck trying to figure this whole mess out. It's not easy and I feel for you.

I don’t know about normal but this sounds similar to Mom's older sisters. The two were a year apart in age and were close friends all their lives. Betty, the eldest, never married and helped raise Norma's 3 children after she was widowed young.
They did everything together until Betty developed vascular dementia in her 80’s. She moved in with Norma but my cousins saw their mom's health decline trying to care for her sister. They then moved Betty to AL and she started to be very mean to Aunt Norma. It was as if she blamed her for her situation and ill health.
Betty was nice and friendly to anyone who visited her but was so nasty to Norma that she rarely visited. She went more out of duty than desire. It was so sad to see these loyal sisters estranged. Betty continued to decline and spent 5 years in a SNF before dying at age 96.
Norma realized it was the dementia but she still mourns her big sister.

My MIL went through this in January. She became extremely mean and hateful to my FIL, accusing him of all sorts of things. 3 things were going on:
1. recurrent UTI’s. In elderly dementia patients that is often your first sign. They don’t get the urgency/pain that you get women do-and if they do, their minds can’t process what the issue is. Their body chemistry changes, paranoia and anger ensue.
2. constipation/impacted bowels. She was not drinking enough fluids because of her incontinence and her brain said loony drink anything so you won’t need to pee so much. 😩
3. She was taking a step down in cognition. This added confusion/stress.
This became the perfect storm that forced us to move her to memory care. She was threatening to leave, wandering at night, yelling/accusing him of imagined harm. It was awful. We addressed the physical issues-more fluids, antibiotics then preventative D-Mannose and Vitamin C every day for UTI’s. The added liquids helps with constipation, and the memory care facility has got her on a more even keel with everything else.

Put her in SNF ...they will be able to handle her...don’t let her abuse Dad again. Hugs 🤗