One correction though: this guy is NOT the security guard, that's another character with whom I've had some issues in the past. This guy, let's call him Z. I can't find any other posts on the forums unfortunately so I don't know his age or profession. My guess is he's a 17-y-o schoolboy, though still with anger management issues. I have my own screenshots from his actions in the guildchat, I have screenshots with more whispered threats to me which he voiced to another guildmember (who also happens to be a colleague). I have screenshots from the lovely things he said in the forum, with the IP visible in the picture.
With his colorful language and repetitive threats, I can link all the shots together so that I can use the screenshots from the game together with those from the forum. That should be enough to scare his ISP into action.
I'm glad now I always read thaily's adventures with trolls :D
Edit: Oh wait, I just realised that I can report him to the Blizzard GM's as well! Verbal Harassment, with evidence. There goes his account. Tempting.