I know I'm not the one you want a reply from, but here we go
The example you provided is vastly different than just saying "Skivvy, you're bad" or something similar. There is nothing wrong with offering advice. I give advice if something is going horribly wrong (I tend to put up with a wide variety of play before I feel the need to speak up personally), but I've never had someone snap back at me or take it poorly. But then I also to tend to be more wordy (like your example), and maybe toss in an emote for good measure. Tone can be a tricky one to convey with text.
From the other comments in the thread, it sounds like people are saying that it shouldn't be offensive to just call someone straight out for being bad. Simply telling someone, 'you suck', or 'you're bad' would both be insults given the context. Both may be factual, but it's still insulting to put it that way.
I think the problem a lot of people have is that they don't realize how they come off in text. Or perhaps they just don't care about offending anyone, as someone mentioned on a page back.. I'm of the opinion that it's really not difficult to show a basic level of civility towards the strangers you're playing with. We're all here trying to enjoy our free time - how about we at least try and be friendly with each other?
Depending on the fight, I know that sometimes it can be hard to type full sentences, and maybe all you can squeeze out is a 1-3 word comment, but it is still possible to phrase things without inducing knee-jerk reactions from the other party.