Some of Mum's traits became caricatured when she got dementia - her parsimony, her hanging on to things so that the house became cluttered, her critical approach to other people's looks, her distrust of people's motives, her wanting to maintain the mother-daughter hierarchy at all costs, her love of contradicting other people.
It was often unpleasant dealing with her and I felt hurt because I could remember incidents in our past when these traits had also been present. It was like reliving old conflicts.
On the other hand, now that she's getting some medication for her dementia (memantine, citalopram) some of her good qualities are reasserting themselves. She was always polite to strangers and now she is well-mannered with the carers. And one new trait has appeared - she has become fond of animals, having spent years being indifferent to her children's dogs etc.
I am finding it easier to get on with the new Mum!
As far as this thread goes - I don't think you can generalise. I think dementia affects people in different ways. Some people get changed out of all recognition, and others get their recognisable traits accentuated.
But I do sympathise with the upset feelings that the behaviour can cause, when it does seem as if there was a seed to this nastiness in the past.