There's not a lot I consider unforgivable sins when it comes to writing and literature.
Some are obvious, terrible, but not necessarily unforgivable. Merely unfortunate: Being boring, mediocre and unambitious. Others I take a firmer stance on. A writer gets many tools at their disposal. Tools that are prohibited to many other professions, such as lying. Fiction and many other forms allow and encourage lies to strike at deeper truths.
Still, even a writer has some responsibilities.
One unforgivable sin for a writer?
I'm going to call out writing to deliberately make another person or even one's self stupid.
I'm not talking about writing that's nonsensical like 'The Jabberwocky' or Vogon poetry. I'm not talking about policies or ideas that are wrong, confusing, vague or uninformed. I'm calling out work that has been written with the clear and deliberate intent of creating stupidity and somehow lessening intelligence within another being.
To have woken up and said, by god, I'm looking for a way to make a complete stranger a numbskull, for whatever purpose, be it commercial, political or some other ill-conceived social gain or even just for the hell of it.
To me, that would be one of the most unethical and unforgivable crimes a writer could deliberately commit. Now, this is not to say that this might occur unintentionally. But, the wanton intent would be reprehensible, the uncrossable line.