For those of you who disagree with the OP's action, you must be concerned about the same consequence befalling you. There is nothing wrong with telling someone you think they are rude. In fact, I have done it, when some dipwad came up and skinned the corpse of something I had just killed while I was still standing there, trying to skin it. I simply sent them a tell saying "It's rude to skin someone else's kill like that, you should at least wait to see if they are going to skin it." I got no reply, nor was I expecting one, at least not anything polite. However, maybe, just maybe, I told them something they hadn't thought about.
A lot of kids play this game. Apparently, quite a few of them have parents with no clue how their kids are acting online. And the anonymity of being on the internet, as also previously stated, can sometimes bring out the worst in people. They think, because no one knows them, they can say and do whatever they want. It's every man/woman for themselves, and I don't care that it took you 15 minutes to clear, I'm gonna run up and kill the named. Sarcasm is not different than rudeness, either. "Oh by all means, glad I could clear for you" is rude. If it's an accident, you didn't see them clearing (yes, it could happen, I've done it) that's one thing. But how do you know it was an accident if you don't say something? Maybe by saying something, they will become more aware of their surroundings.
If not, oh well, at least I tried, but if I could do what the OP did, you bet your butt I would. I know quite a few people who play WoW, and their kids. I don't think they'd take kindly to hearing that, because I told their kid I thought they were rude, they started swearing at me and questioning my sexual preferences. I'm pretty sure they'd be pissed, and if it was my kid, they'd be banned from playing for at least a week. At least. Anonymity is no excuse for being an asshat. Karma will get you, eventually. In this case, it just happened sooner than later.