Mom's angry, negative and mean. Not sure if its because she's in pain from arthritis or if it is some form of 'angry' dementia?

Sounds like my mother as well. She just lies in bed, unhappy with most things on TV, but not willing to do much else. I know she's depressed, but she doesn't want to deal with anymore pills or doctors or hospitals. Is your mother willing to be tested? Maybe something as simple as St. John's wort might help. Is she mobile at all? Maybe getting out and walking would at least give her a change of scenery. (My own M can't get around anymore.)
I agree, your father could use support as well, don't overlook him.

Daughter can you give us more info about your mom - her age and general health. What has she been diagnosed with? Does she have dementia for sure, or are you just thinking that might be going on? What's her health history? Tell us more about her so we have more to go on.

It may indeed be the pain, or it may be depression. Just as Dustien said, when you don't feel good, it's easier to be nasty.

She is not happy, and it's possible that the right medication could make her happier. Do what you can to get her doctor to address this problem. Maybe you could write a letter to the office, telling him/her what is going on, so no one would have to discuss it to your mother's face. I think this is also a risk to your father's health to live with all the stress. Do what you can. Good luck!

My mom was very much the same way for months leading up to a diagnosis of Lung Cancer, the cause of her death a few month's later. Don't blame your dad for not wanting to rock the boat. My dad had no choice but to act as your dad is acting...just go with the flow and try not to "rile" her. What else could he do? After 56 years of marriage it's not like he can up and leave her.

We're thinking that the cancer, when she didn't know she had it, had been making mom feel "under the weather" for a long time. She probably just chalked it up to old age, but it had her feeling grumpy and she was being overly snappy with my dad all the time. Kinda like a lot of us...we're pretty nice until we get to feeling bad (flu, cold, bad headache) and we find ourselves being snappy without really meaning to be.

When my mom was finally diagnosed with cancer she became so moody that the doctor prescribed meds to calm her anxiety. They evened out her moods and had the added benefit of making her nicer. You might talk you mom's doctor about what's going on with her, or have your dad talk to them. Hopefully it's not something as serious as a lurking cancer, but this might be a way to even out her moods and give both her and your dad both a better quality of life.