People get angry, and sometimes they need to vent that anger. There's nothing inherently wrong with being angry, just like there's nothing wrong with feeling any of the feelings a person can have. There's not even anything wrong with expressing that anger. The body keeps the score, and it will not forgive someone for their unprocessed emotions. Expression is often a part of that processing.
Everything after this is my opinion and should be taken as opinion, not any sort of expression of fact.
It is my opinion that an angry statement like "ACAB" isn't harmful as long as the audience is right. I have no issues with someone saying "ACAB" under an article about police brutality. I would be displeased with an ACAB comment as a direct response to someone who is a cop and was expressing vulnerable feelings about being a cop. It's been my belief and experience that that type of moment is where the right words can actually change someone's mind a little bit, whereas the wrong words can cause harm and push someone to double down on their beliefs. The only times I've managed to change minds is when I first try to express some degree of understanding before attempting to reverse the connection to show the other person why they would benefit from understanding.
I do see the irony present in my specific example here, since bad cops generalize and act out those generalizations on individuals in painful (and sometimes lethal) ways. I am not able to feel or act that way, so the bad cops get to do something that I can't. ¯\_(ツ)_/¯
Basically, as long as a commenter is venting at a distance, I think it's relatively harmless. It makes me unhappy when a human is being directly and aggressively dumped on, because the chances for real harm are greatly increased.
I don't know if I feel quite the same way about "muskrat." Like, that's very much about a specific person, but the dude is literally never going to read that comment. I very much dislike Elon Musk and usually don't say his name in conversation without the word "fucking" in front of it. I personally don't believe that an internet comment can ever make that man act in a better way, so I don't know that there's any point in trying to treat him nicely. I dunno. Feelings and shit are hard.
EDIT: I should also mention that ACAB is not an insult, it is a symbol of a movement. That symbol can be insulting to some, but it can also provide a sense of solidarity to others. I am a very privileged person that hasn't had bad experiences with the police, so my understanding of the anger behind it is the product of empathy and relating it to my own traumas. I would not personally say that to a cop in a situation where I might be able to change their mind a bit, but I haven't suffered from oppression and pain at the hands of the police. Props to themeatbridge for their comment, because it made me realize that I was framing ACAB as a pejorative rather than what it is.