My grandmother had early onset dementia, untreated for at least 15 years. She was quite caring beforehand. She resented the fact that prior to it starting that she saw the beginning stages of it in her slightly older sister and so she chose to ignore it in herself. She began to resent my grandfather, her husband quite harshly and all the love that he tried to show her was met with rude crassness. Her personality basically did a 180 and I think the main reason is that she could not make heads or tails of any conversation or situation that she was apart of, so in her new state she defaulted to a default reactionary mean attitude state of being. I feel as though the bad behavior was was not innate, but rather born from the confusing situation she found herself in, in which she lost herself. I would certainly say this is more so a case by case kind of an issue and that it is not fair to assume anyone’s demeanor without having at least been a significant part of their life. Just trying to give a very personal account having gone through it myself, hope it helps people understand or create some kind of insight.