Tips for writing a good "insane" character and avoiding being ableist
Two types of mental breaks/dangerous people will be covered here
Those that are dangerous to themselves and those that are dangerous to others
Mental vs Psychotic Break
If the character is dangerous to themselves they might be more withdrawn and untrusting
Stress that a mental break can be extremely scary for the person experiencing it and that they dont mean to hurt anyone
They're more likely to be avoided and isolated than anything
If they are institutionalized you should make sure they arent forgotten about/tossed to the side
Study causes and signs of minor breakdowns
Now read memos from people who actually had a mental breaks
Avoid offensive language and slurs
DO NOT use them as some oooOo scary edgy character
Mental breaks are more tragic than entertaining
Something should trigger it/it shouldnt come out of nowhere
Now if the character actively hurts others
They are more likely to be charismatic and blend in at first
Stress that they are CHOOSING to hurt people
Avoid labels and hard diagnosing them
Just let it be "this person experienced fucked up things and decided to become an asshole"
Don't ascribe some big meaning or message to it
If they are a murderer then study murderers and their MO's if you have the stomach for it and want to do some research
remember "the Federal Bureau of Investigation (FBI) states that the motives of serial killers can include anger, thrill-seeking, financial gain, and attention seeking." (wikipedia)
AVOID GLORIFYING THEM/THEIR ACTIONS
Make them an asshole, make them snobby and unlikable because that's what people/characters who actively harm others are