Harsh speech and insanity

Sometimes the idea of arguing reminds me of xkcd: Duty Calls (which kindly mocks that practice).

Conversely I think that Buddhist doctrine is more like this -- i.e. "Others will [...], but we will [...]".

A young Quaker -- i.e. a "pacifist" -- once told me, "I'll discuss anything with anyone; but if it turns into an argument then I [...] and walk away." I think that gives you an option: to discuss, or maybe comment on a topic (perhaps to offer discussion) -- without a counter-productive argument that escalates into harshness. When I think about "harsh speech" that I've very occasionally seen in comments on this site, I think that one of the remedies is:

1. To say what you need to say (e.g. to comment on an answer) but preferably without being harsh.
2. Someone might then reply to that comment.
3. At this point, don't forget you have a choice:
   • If your reply would continue a friendly discussion then why not
   • If your reply would be an argument then maybe you shouldn't -- you said what you had to say in your first comment, maybe leave it at that.

There may be a difference between in-person "speech" and on-line "writing" but I'm not sure what to advise about that.

I can't easily reply to the part of your question about insanity.

My experience of schizophrenia is of years of thought disorder (both, "content-thought disorder" and "formal thought disorder"), as well as of years of lucidity. The question (the OP) seems lucid, it's not a word salad, and my experience may not be applicable to your situation.

Even so, for what it's worth, my experience as a participant or care-giver is that it's essential to remain kind.

• Discussion can be frustrating (if you're trying to persuade someone who won't "comply").
• It can also be disorienting (to experience a disconnect in the "meeting of minds" that you might normally expect in a discussion).
• Sooner or later it might be tempting to try something else -- e.g. "if kindness doesn't work then how about harsh speech?" -- but I think that's most likely a mistake, and you need to find another option (maybe talk with someone else instead).

One last piece of advice is, I think that maybe professionals don't indulge (don't participate in or encourage, but maybe also don't dispute) a person's fantasies, but instead interpret what they say in a way that keeps the conversation on "common ground".

For example, hypothetically:

• Psychiatrist: "How are you feeling today?"
• Patient: "I am the Supreme Being."

In reply the doctor needn't discuss the patient's statement that they are God, but might instead reply with something like, "So, you say you're feeling good."

Can you advise on how best to work on that?

I think., you have stated the reason yourself as below.

I have not taken any precept that prohibits it,

Taking the precept is the best option that I can see.

but then it is my default bad karma.

Intention (action) is not a karma vipaka (result of past karma). Intention is karma that gives a future vipaka. Taking or not taking the precept is not a karma vipaka. Taking the precept is a wholesome karma that gives beneficial results in the future. Not taking the precept is an unwholesome karma that gives harmful results in the future.

I am often harsh, though I don't think hurtful, in speech about others, because of my schizophrenia and psychosis.

I guess Metta meditation would be a good remedy.

I often believe they are up to no good and need to be stopped verbally from doing that.

The Buddhist teaching is the world is not under the Authority of any one. Everything in the world is Anatta (devoid of hegemony). We can't control the world or change the world according to our wish. World is owned by causality. World functions according to the Dhamma-formulas.

Understanding the nature of the world makes our mind to work according to the Dhamma-formulas. Then we would not try to change the people in unskillful way. We can learn skillful ways of advising as well.

If you could read Nandivisala Jataka, then you would find the teachings on the drawbacks of harsh speech and obvious benefits of friendly speech.