I agree with virtually every word you have said and appreciate your reply. As a music fan and parent of 2 musicians/actress, I think it is an appropriate analogy. I also believe in freedom of speech with the proviso that the underappreciated right to it should be respected by not abusing the right. The OP posed the question whether some of the posts went beyond acceptable and doubtless you will agree that what is or is not "acceptable" is subjective and wide ranging. I do think that some people back off venting abuse at matches, not to spare the team or players but to avoid a negative response from others around them ie they are afraid or embarrassed. On a public forum the same people think they are untouchable and have no responsibility or restraint in the level of abuse they dish out.
To return to the music analogy, in my opinion saying something like "when I saw him he was crap" is on a very low scale of abuse and if you read the OP again I doubt that that is what the question referred to. Some posts on a number of threads go way beyond performance "criticism" , nothing to do with slagging as generally understood, and have nothing to do with football. The moderators may think differently or may not be aware of some of the stuff, but I think the OP is entitled to raise the issue as a topic of reasoned discussion. Performance criticism is one level but should posts that veer towards personal abuse, potentially defamatory statements not based on facts and unrelated to the performance be acceptable ?.
So after seeing the musician not perform to the level expected you are "emotional" and you say in the moment "when I saw him he was crap" do you then go on his fan forum slagging the performance, saying he should quit as he was shit, then make references to his personality, his gender, his looks, religion, politics, race,drug habits, his family background etc. ad infinitum then repeat it 2 days later and if anyone was unhappy about it tell them to "fuck off it's a free speech forum" . Is that acceptable when saying the same things at the performance is something you would not do ?
If the performer was your relative or friend or you were a devoted fan, would you still think that reading such a post, if posted by another fan, would be fine "sure it's only free speech" ? I guess we have the rest of our lives to answer these type of questions, if we have a mind to.