It's disrespectful. The guy died in real life, doesn't matter if it's an in-game funeral or not. If there was a thread for well-known head-fier who passed away, would it be alright for people to intentially troll in it?
(I don't even play online games, but I still consider the attack inappropriate)
I don't play them either, but I can see why they'd be upset. If that's the way they knew the guy, then it's cool that they decided to do that. If one the the members of the attackers group died I bet they'd be pissed if the same thing happend.
It's the internet, people can be jerks without reprecussions. Besides, that clan or guild or whatever it is in WoW could have had the ceremony on a protected server of some variety (so I was told), but chose not to, having it on a public server, of some variety. I obviously don't play those sort of games so I'm shy on the technical details.