How to deal with anger and bad attitude?

My mom 78, has dementia. She's in a home, a small group home with a few other ladies. For a while she was good, but lately she's very angry.

To get her out of her apartment, the woman who runs the home told my mom they needed to keep an eye on her for a few days, at least. It was the only way we could get my mom out. Any other way probably would have involved calling the police, etc.

Of course my mom doesn't understand what went on. She just thinks it's trickery to steal her money. Before she lost her right to live alone, nurses, an attorney, a doctor, social worker and more evaluated her, and then it went to court. In court she didn't understand what was going on. She always goes back to the trickery, that everyone did it to get her money. (Not that she had a lot.)

Now all she does is be bitter: I visit her, and she tells me she could have me thrown in jail for what I did. She says she's going to sue and get the state to pay her for selling her into slavery. She complains about everything from the food to her roommate and so on.

Question is: How do you cope with this? I try to visit her each week and call a couple times a week, and I loathe talking to her now. For a while it was OK. I know she's taken care of, which is good, but all she does is complain, which is draining. When I visit I try to do things she likes, taking her out to lunch at a place she likes, or playing games with her, or shopping a bit for/with her. I bring her things, too, that she may like or need.

But all she is nowadays is angry and bitter. How do you cope with that? I can't reason with her, obviously, but it's more a chore to see her now than a pleasure. Everything sparks a reason to complain or be bitter, it seems.