Wilhelm died, but anyone who's played World of Warcraft knows that death isn't a permanent status for the inhabitants of Azeroth. You simply need to walk over to your corpse, and you're as good as new! Of course, non-player characters aren't afforded this same luxury. Wouldn't it be awkward if Arthas just respawned with slightly less durability after you killed him? I guess it's best not to think about the logistics of massively-multiplayer games.