Like I explained I was trying to not spend a lot of time with long text in this case, which I think is the main reason it ended up sounding rude.
Your quoted line is not what I wanted to get at at all and I don't really see how you read that out of the chat. The point I wanted to make was
"Don't intentionally hold back the party, and if you don't want to do otherwise on the job you are currently playing, then you are better off choosing a different job to avoid other players becoming frustrated with you". And he would most likely enjoy that way of playing more.
What I get from the replies so far is next time, I'll try something like "I noticed you are doing X, but the job can be played much more efficiently. I can try to give you advice, if the party gives me a minute to explain."
For answers like "But I don't like to do it that way", I'd reply "Give it a try. If you intentionally hold the party back, many players will get frustrated with you and I'm sure you don't enjoy that. If you don't like playing the job that way, maybe consider trying a different job."
Now that took me a few minutes to come up with, to be especially careful not to come across as rude, and in the case I already mentioned, I'd have been kicked for "AFK" by that point. So.. I guess I better macro this. For me, while I realize what I said to this BLM might have sounded rude, the amount of information contained in "You should choose another job" (I just realized this english is wrong..) and the above which took much longer to think about what I'm writing is pretty much the same. I find it's not easy to find a sweetspot inbetween these, and, even if it sounded rude, if the answer wouldn't have been directly insulting, he might have found out I didn't actually want to insult him either. Instead he chose to call me an ass an leave the instance as quickly as possible.
I understand that, but I think this is a highly disturbing fact, and one point I wanted to get across by making this thread is that, next time you think someone is being rude, don't be rude back, but talk in a friendly manner. I think, many, many, MANY times in the end you will find out that person was not trying to be rude at all. I try to make this effort, and I think everyone should. If you don't want to bother, please, just ignore me, but don't insult me because you thought I was being rude, when that wasn't my intention.