Don’t dress up as mental patients at Halloween. Mental illness is not a costume and you’re demeaning and mocking severely mentally ill people when you make mental patients into a scary/funny costume.
Don’t dress up as psycho/insane/crazy killer for Halloween. Doing that will reinforce the association that mental illness and violence is related, a myth which harms mentally ill people with stigmatized disorders like schizophrenia, personality disorders and bipolar disorder every day.
Don’t make a “mental asylum” themed Halloween party or haunted house. Doing that is making a mockery of all the mentally ill people who have suffered and been abused in asylums in the past and those who are still going through psychiatric abuse today.