Directness isn't mean. If you go into an argument and think someone's argument is based on premises that harms their argument from the beginning, you tell them. "You have three fatally flawed basic premises" is a neutral statement. There really isn't any emotion, niceness, or lack thereof attached to it.
Firstly, I would think it's because the poster whom the prase was adressing didn't even comment on it, it was another poster, and they added needless aggresiveness to their posts by strawmanning, mis-quoting and belittling other posters.The fact that the author of this sentence, or the ones that defended him, didn't try at all to say something like "I apologize, I didn't mean to sound rude" but say instead things like "you don't know english, stupid", says a lot.
Secondly, you are strawmanning yourself. Noone told candybarsuvius "you don't know english, stupid". They explained that they view "fatallt flawed" as a neutral, commonly used expression.
Like Kish did?That's not something I would tell to someone unless [...] I want to show my strong disagreement with it.
What's humourous about your sentence is the added death threat which removes it far, far away from anything expressed by Kish in this thread.Like in "It's a fatally flawed opinion to think that you can steal my cake, and leave this room alive" - see that? These two words are so strong that they can't be used in a normal conversation without being humourous.