I believe an aggressive comment in a forum discussion would be when the target is perceived to be the individual, and not the content of his comments. None of the following are quotes of anybody, just examples.

"You're stupid" - clearly aggressive

"You're stupid because you don't know that foo() returns a hashref in scalar context" - aggressive

"No, that won't work because foo() returns a hashref in scalar context" - not aggressive but might be perceived so depending on the facial expression and tone of voice of the speaker, as assumed by the reader