A derogatory credit letter is generally not something an individual would want to write, as it can seriously damage a person's credit score and make it difficult to obtain credit in the future. If there are any outstanding disputes or issues with a credit report, contacting the credit bureau directly to resolve the issues is advisable rather than writing a derogatory credit letter.
If there is a legitimate reason to contact a credit bureau to dispute inaccurate or false information on a credit report, it's best to do so in a factual, clear, and professional manner, avoiding inflammatory or damaging statements that could further harm your credit score.