The case you've mentioned sounds like the person was experiencing psychosis. I can offer you a story about my own experience with psychosis in order to explain how it feels, so that it might help you.
I've always been described as sensible. I was always mature beyond my age group, reliable and predictable. I hit puberty and developed clinical depression, which had psychotic features. I was delusional and thought my family was trying to kill me or abuse me. I had false memories in my head about abuse that never happened. That was the mild end of my psychosis.
I went to university and I was still undiagnosed. My school had picked up problems but a psychiatrist passed me off as just a "problem child", which was insane because I'd always been a perfect straight A student, head girl type of person. But he saw how irritable and hostile I was and decided that I was just bad, not mad.
So, uni. I started to hear my flatmates whispering about me. It didn't stop. I lay in bed and knew they were saying I was weird. They laughed about me. I'd try to catch them in the act, but every time I looked through the pinhole, they'd gone away. I messaged my boyfriend and told him I had to get out of there and that they all hated me. It did not occur to me that I was hallucinating because it seemed like absolute reality to me. I was alone in my room, so nobody could point out that there were no voices and everyone just believed my reports that my housemates were horrible.
I moved to another uni and my mood settled for a year or so. I was studying healthcare. I started to feel inappropriate guilt about a patient who'd had a cardiac arrest. I went on holiday with my boyfriend and found myself curled up in a ball inside my tent, crying my eyes out.. over nothing. I didn't even feel sad. I actually felt nothing at all. I started to become paranoid that I was a psychopath. I dumped my boyfriend very abruptly and started sexting his friend, something I'd never have done previously. I thought I had no soul and that somebody had taken my soul away, and I got out books from the library about psychopaths, and I told my ex that I was one.
I started feeling the presence of the cardiac arrest patient and I was convinced that she was haunting me, but I didn't believe in ghosts. I ordered my ex to move out of the house and thought I'd be better off alone. I felt very uncomfortable around other people, a sort of weird edginess.
I went to a counsellor at the uni and tried to subtly suggest that I was a psychopath so she'd tell the police and they'd lock me up. I thought I was dangerous. I told her that I'd been faking a personality my whole life and I wasn't real at all. I told her I was evil. I began to slip into a weird fantasy world and I watched documentaries on psychopaths and wrote long, erratic essays about how cold and unfeeling I was.
One night, I got images in my head of strangling my cat, so I locked myself in the bathroom and screamed and cried all night because I didn't want to do it. I went to the counselling centre the next day crying my eyes out because I didn't want to murder her. I got referred to hospital for a psych assessment and the nurse on duty decided that I was just upset because I had an essay due for uni.
Also, a few months previously I'd submitted an essay about church burnings in Norway in answer to a totally unrelated question. It just came back with a "?" on it. I was convinced I was going to get an A. It was a totally absurd piece of writing, looking back on it.
So I went back home and the ghost was fully-fledged by then. I saw and felt her everywhere and I wrote the Lord's prayer on my arms in Sharpie and muttered it constantly to ward her off. I didn't want her to hurt my cat I glued salt to my hands and put it in the doorways to ward off spirits. I wrote totally mental stuff all over facebook for anyone to see.
I went back to the counsellor and she got me a referral to the crisis team because by that point I'd begun hearing voices all the time and I'd confessed that the goldfish in the waiting room were telling me not to trust the counsellor. The floor was breathing and moving into images of the devil and I kept hearing people saying "she's going to tell them, she shouldn't be doing that!". I was constantly crying but insisting that I felt nothing and I was evil.
I said I was going to kill myself, because I was living in an absolute nightmare by that point. I'd cut myself a few times already. A psych came to my house and I stared out of the window, talking in a flat monotone I didn't recognise, and she said I had schizophrenia.
I went to hospital for a month and was put on a high dose of antipsychotics. Hospital was a weird experience.
But I hope this has shown that these things are insidious. This took over across many years. I was, and still am to some degree, a "logical" person. But I was still in a position where I was sitting on a hospital ward, bawling my eyes out because I thought one of my old patients was going to skin my cat alive. And the healthcare assistant said "you're scared of some old lady?" and I realised that she was just some old lady. That what she'd built up into, in my head, was not real. But I couldn't wake myself up from it without someone else's help. It totally took me over.
Wouldn't wish it on my worst enemy. Sorry this was so long. I missed a lot of stuff out as well (suicide attempts, one by crashing a car, were quite awful).