The etiquette policy is codified in the FAQ:
Civility is required at all times; rudeness will not be tolerated.
Treat others with the same respect you’d want them to treat you. We’re all here to learn together. Be tolerant of others who may not know everything you know. Bring your sense of humor.
Above all, be honest. If you see misinformation, vote it down. Add comments indicating what, specifically, is wrong. Provide better answers of your own. Best of all — edit and improve the existing questions and answers!
When you stumble upon a comment you find rude / offensive, you should flag it as such and walk away. From your question I gather that you responded to the comment, and that wasn't really necessary, if someone is clearly having an off moment there's absolutely no need to continue the discussion.
If other users flag the comment as well, it's possible that it will be deleted automatically, if I'm not horribly mistaken it takes 3 "rude or offensive" flags. If not, the moderator responding to your flag will either delete / edit the comment or dismiss your flag, if (s)he happens to disagree with you.
Whether there will be any further action or not is completely up to the moderator, it's a judgement call and there are a lot of factors to consider, nature of the comment and past behaviour of the offender being the more important. Generally speaking the moderator has three options:
- Do nothing (a perfectly valid option in some cases),
- Send a private warning message to the offender, or
- Suspend the offender.
Regardless of what the moderator decides to do, the matter now is a private one, strictly between the moderator and the offender.