How can I deal with an abusive, hostile, extremely demented 91 year old man?

Helen, I think you are on the right track in thinking about a medication that will calm him down without knocking him out. Your father will likely retain the negative energy field around him, but it may calm him down enough that he isn't as bad as he is now. You are in a position that it's hard to know what to do. Taking conservative steps sound like the way to go. Maybe the doctor has some good ideas about what medications to try.

Don't feel guilty that you don't want to spend more time around your father. Being in that negative energy field is so tiring. My mother has one of those fields. She usually isn't mean, but she complains a lot or talks about all the people from her childhood, e.g. her cousins with strange names. Spending an hour with her wears me out like a day of digging ditches. Strange how it can have such a big effect. My answer is that I do the things that need to be done, but when I feel I'm being drained, I find something else I have to do. Then I disappear. I can feel guilty, but I don't owe my mental health to anyone.

If he has dementia, then, I wouldn't characterize his behavior as if it was intentionally cruel. Even if he were intentionally abusive before he got dementia, he can't be blamed for his conduct now, if that is what is causing it.

Has he been actually diagnosed with dementia? Why can't he be admitted to a psychiatric hospital to be evaluated and treated? Is he a threat to himself or others? There are procedures for that. You can consult with an attorney or law enforcement officer for your options.

Ignoring violent or threatening behavior can be very risky, even if it is due to dementia. Not long ago something very tragic happened in my community when a husband/father who had dementia was not properly addressed in the home. He killed his wife and daughter. He was arrested, despite his condition, but, it's not clear that he even knows what he did. You can't play around with this type of situation. Safety first, imo. I'm sorry you are having to deal with this.

Helen, you do face a dilemma, but your mother's health is important, as is yours. The investments should be used for their care, whether it's for medical, housing or other help. And getting him into an appropriate placement might actually prolong your mother's and your lives.

My parents have some investment $, so would have to pay full price for a nursing home. He is physically healthy & could be there for a long time. Mom would have to give up her house & take on a whole different lifestyle. We are at a loss as to what to do.

Protect your mother and yourself; find a placement for him. Verbal and physical abuse is not to be tolerated.

It sounds like it's time for you and your mom to have some peace. Have you considered placing him in a nursing home?