Don't pay no mind to those asshats.

You hurt their feelings by going your own way and they took it as a rejection; you dumped them. I'm guessing their thinking is if you're not WoW'ing with them then you're either quitting the game period or you're dead.

That guy who posted on the guild forum is not emotionally mature enough to simply wave goodbye and tell you how much fun it was to have you with them. I don't care how old he is, and by your description it sounds like others in the guild harbor those same feelings which makes them just as emotionally immature.

I guess there is also a bit of gang mentality to it as well.
I've seen this same mob mentality wreck havoc throughout a guild; the gm gets mad, then the officers, then the guildies. It's like a floodgate that has been opened. You were honest and cleared your conscience. That's all you can do. TBH, if they act like this at someone's leaving then I'm sure theirs is a guild I wouldn't have liked being part of.