Is There a Hippocratic Type Obligation to Write a Bad Rec Letter?

It's recommendation season and as I write a number of rec letters for former students and colleagues who are very much qualified for the graduate school and/or employment opportunities for which they are applying (don't worry A Stranger --it was a good letter), I can't help but wonder about having to write the negative rec letter.

I'm not talking about the case where the student is not qualified for the position. Of course, the proper thing to do is to politely decline to write. "Given that you received a D in my logic class, I don't think a letter from me will be very helpful in your attempt to get into law school." Those students, by in large, are not going to be admitted, so your lack of a letter is not really the operative factor.

But what of the student who is qualified in terms of the standard measures (grades, board scores,...), but just is the absolute wrong person for the job? Suppose someone has the grades for medical school, but you know they lack the compassion to be a good doctor or social worker? Or the convictions to be a good lawyer or climate scientist? Suppose you have good reason to think that he or she would use their advanced training in a way that would harm others or the planet? If you don't write the letter, someone else will and the person will become the person you are worried she or he will become. You can help stop it. Should you?

By agreeing to write the letter, aren't you implicitly agreeing to write a supportive letter? Doesn't the community ethos lead the person to expect to be told that you cannot write a letter in strong support? If you are using this as a chance to torpedo their application, isn't that misleading in some sense?