I knew someone who lost a parent to dementia. They learned that if the person was nice in life, they were nice when losing it. Also, if someone was a mean bastard they were usually worse.
I lost my wife to early onset Alzheimer’s. You are not correct. I lost my mother to dementia. You are not correct.
Sounds like you have one data point. Pretty hard to connect one dot.
How they were has no bearing.
I have talked to a lot of people about living with a parent who has dementia and it's often blamed for changing their personality.
But I have always suspected that it never really did, that what it did do was to remove the inhibitions a person had from acting on what they were really like in their heart. The real them comes out.
And it's usually not pretty when it does.