You can direct a post at anyone you want, but posting in a public forum leaves it open to public feedback.
You can refute a statement/assumption without using insults and without going into great detail about your assumptions of anyone's intelligence or motives. And no, 'he started it' isn't a valid defence, even if accurate, especially if you are escalating the situation.
The more you do focus on things off-topic, especially written in a way likely to provoke a reaction, the more off topic the discussion will become.
