Yes, dementia can be horrifying. My Aunt Elizabeth had what IIRC is called "sundown syndrome" where they are fine most of the time, and then there comes a point in the day where they lose time and forget what is going on. Yes, that is very mild compared to what you are describing, but it was utterly chilling to me to visit her and start talking, and then a minute later she would repeat the exact same phrase in the exact same intonation (shudder). I couldn't bear visiting her after that; I wanted to remember her from when she was whole, not this shadow.