I have to admit I was really disturbed by a thread that was recently locked. What bothered me so much was that people were suggesting to purposefully frighten a person who had an anxiety disorder.
Now, there are a few things to consider. Yes, some mental illnesses make people extremely difficult to deal with, even to the point where that person can be considered toxic. Some mental illnesses make people so selfish, that they do not care about anyone other than themselves, and "etiquette-proof" — they will never be polite or return courtesies. Sometimes they either don't, or can't take subtle hints. And much of the time normal bounds of etiquette simply do not apply.
That said, there is a kind way to treat even the most toxic mentally-ill person. Granted only rarely there is any appreciation or reward from the mentally-ill person even when he/she realizes that you've been kind despite their rotten treatment of you, but that shouldn't matter. If we know better, than shouldn't we be polite anyway?
Here are my "rules of etiquette" for dealing with someone who is mentally-ill (I am assuming that either the person is not toxic to be around, or cutting them off is not a choice):
1. Do nothing to exacerbate their condition. If they are paranoid, don't lie or play tricks on them. If they have an anxiety disorder, do not purposefully freak them out or scare them. The only exception is if you are their physician and they are voluntarily undergoing treatment.
2. If a third party must deal with them, it is fine to give that person full disclosure on their behavior, but you must also mention their diagnosis. Mental-illness does not make a person "bad" even if their behavior is bad.
3. Do what is necessary in your power and right to keep that person safe. Do not attempt to step on their legal rights unless they are in imminent danger.
4. Preserve what dignity you can for them. They are people too.
5. Even if they drive you crazy, try not to take it out on them. It's not their fault even if it is their problem.
Any others? Thoughts?