If you need torture ideas, just read about the way mental patients were treated in the 19th century, such as in Bedlam.
Yeah, and you don't even have to go back as far as that to find some pretty horrific stuff (USA specific here, I don't know as much about the history in other countries). Lobotomies and shock therapy performed on the LGBT+ population (as recently as the 1940s:
http://www.huffingtonpost.com/jamie...-gay-aversion-therapy-revealed_b_3497435.html), on women who were too willful or unladylike- and look at the way that people used psychiatry to control and terrorize people of color, particularly black people, in the USA. They used to say that black slaves who tried to run away suffered from "Drapetomania" (since it went against the supposed "nature" of the enslaved to try and be free). To this day schizophrenia is disproportionately diagnosed in black men and the roots of that come from the civil rights era (a good article on the subject:
https://www.psychologytoday.com/blog/witness/201011/how-the-black-man-became-schizophrenic)
Basically there is a lot of nasty history with psychiatry and, honestly, there are still ongoing problems today. For instance, some mentally ill people are very vocally against the very notion of an involuntary psychiatric hold, both because of the abuses in the system- I knew a girl who was sexually assaulted in a psych ward and suffered PTSD around psych wards because of it- but also because it's a terrifying thing to have so little control over your own life when the state deems you unfit to wield such control.
All good stuff to be aware of if you're writing horror along these lines, and potentially a well to draw ideas from.
It'd be nice to see more horror that is sympathetic to the patient and their plight as opposed to demonizing the mentally ill TBH. The horror genre is way to enamored of using the mentally ill as monster figures/ killers/ etc.