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.
Any flag on a comment will put it in the moderator queue.
If you feel it is sufficiently obvious why you find the comments offensive, you may use the "rude or offensive" flag. The advantage is that if enough people flag the same comment, it is auto-deleted and the mods don't even have to get involved.
If you feel that more explanation is warranted, you can use the "other..." flag and include a brief comment. The disadvantage is that special flags don't pile on like the offensive flags do, but the advantage is that you can direct mod attention to entire comment threads this way.
People are expected to behave with common courtesy toward others on this site regardless of the topic, so it doesn't matter why the person is attacking or insulting. If someone is being rude, please don't hesitate to flag it as you see fit.
