My wife's father was recently diagnosed with dementia, and I was not aware of the variety of ways it can affect people.
In his case, the onset was quite rapid, and his primary symptom is chronic horrible delusions. His family ended up calling 911, because he was freaking out, and they were afraid he was going to have a stroke.
In the emergency room, he would tell everyone he met about all the people he had killed and was going to kill. He's going to commit suicide, he doesn't believe in God, he IS God, he's going to hell. He thinks the hospital is going to cut off his arms and leave him helpless and in pain. It's just an incessant stream of horrible nonsense.
It has been an ongoing process to see if there is any combination of medications that can improve his quality of life. There has been some improvement in his anxiety level, but nothing so far has been able to stop the delusions, and his general mental capacity continues to noticeably decline.
Ugly stuff.