It's all in the phrasing. I find commonly people will forgive you for taking a few seconds to type a well thought out critic, as long as you are gracious and make sure they understand you're only giving advice. I have advised several people on jobs that I play while on one of my other jobs (not tank though, god I'm an awful tank haha), and most have been varying degrees of thankful. You just have to be willing to take a moment to make your intentions clear. Believe it or not, wording does matter. People read as much into the way you talk as what you say. Incomplete sentences and poor spelling come across as glib and uncaring, even if the writer never intended them to seem so.

Also, I don't think they took offense at the first statement as much as your pushing the point. He didn't agree (incorrectly, but that's beside the point) and you pushed the point. You can't make someone play your way and not sound a little like a dick, if you're forgive me taking his word.