Remember that there's a human behind those posts. If you wouldn't say it to his face, don't say it to his avatar. Keep in mind that the internet dehumanizes every interaction, encouraging bad behavior, and err on the other side. My two cents, since you asked. (Of course, two cents doesn't go very far these days...)
Keep in mind that there are some folks who live to post things behind the anonymity of a internet forum. We aren't talking to people's faces. We're speaking to the persona they have chosen to present.
Just my two cents from a different perspective...