Darnassus seems to be more difficult than other places, with a higher failure rate for "hearth kiting". Weird random stuff seems to happen when you try it.
I tried the same thing with a teleport to Theramore and it seemed to work. Arthas is defeated very quickly (in this arcanation he has 7900 HP) by the Theramore guards, he doesn't die but he isn't attackable afterwards, though it is kind of cool to have arthas just standing around in Jaina's tower.