I can find no guideline in the FAQ regarding when to flag someone for a comment that is offensive. I cannot the topic in the meta section.
Consider the case when someone is attacking / insulting, and their 'premise' is an incorrect understanding of grammar. (e.g. long comment chains about how a word 'should have been' in a past-tense form, when it was used in the present tense).
Hopefully an answer can address the mechanics of how flagged items are moderated.