How To Portray Our Character (individual research)

How to show a character has personality disorder, making it seem like a psychological horror movie:

Our narrative of the story raised concerns for our group on how to portray our movie as psychological. I've decided to do some research in order to find out how we can use some techniques that they use in the movies to make our character seem like she is
psychologically damaged.

In 'A Beautiful Mind', a patient with schizophrenia sees delusional visions and is shown being harassed by several other characters with who look the same but have weird personalities. We could use this in our product to show that our character too has a problem, just not schizophrenia. We could use the idea that they see delusional visions, but maybe as a nightmare instead of while they are awake. This will allow our target audience to recognise that there s something wrong with our character.

Another example is in the movie 'Split' (trailer below).
The main character (or shall I say 'characters') has several different personalities, thinking that they are someone else after some time. One even pretends to be a 7 year old child. We could use the idea of pseudo identity in our movie because our character will not realise that she has killed her bullies until she sees something that makes her realise it was her, perhaps some news on the television with CCTV footage of her running away from the crime scene, but she will hae no memory of this.