Bad letters

This was posted by Anonymous on the previous thread, and warrants attention:

Wow. So I just read a letter of rec for a job candidate (I'm on a SC). Absolutely brutal. It wasn't just unenthusiastic; it outright insulted the candidate along numerous dimensions. Why would you agree to write a letter if you are going to go out of your way to say that the person is bad? I understand not going in for hyperbolic praise. I read tons of letters like those where it is clear that the letter writer isn't giving full support. But a letter this damning is a rare find.

• Were the other letters equally bad? Was this just one rotten letter from someone who really did not like the student? (One must wonder, as well, why the student didn't know this person thought so poorly of him/her. Was s/he being set up? Was this a personal vendetta? Sexism/racism/bias?)
• What moral/legal/professional duties do SC members have in such matters? Are they prohibited from writing to a candidate and saying, e.g. "One of your letters really stinks -- maybe you should ask someone to look into that." Or some such.
• What moral/legal/professional duties do letter writers have to the persons for whom they write letters? If you are asked for a "letter of recommendation," that seems, to me, to imply that, at minimum, you are going to recommend the subject of the letter. Insulting, demeaning, trashing, and not recommending would seem to be excluded from the mission of the letter of recommendation. Which is not to say that you have to offer a complete and unqualified endorsement of the student, but you have to endorse them to some extent. It would seem that this letter writer engaged in deceit and intentionally undermined the student under the guise of "recommending" them, unless they actually told the student that they could not and would recommend him/her. But then why write the letter at all?