Surely the question should be whether a reasonable person would feel threatened by the message, supposedly humorous or not? I mean, there's not much difference between

"I know where you live & I'm going to kill you & rape your kids"

and

"I know where you live & I'm going to kill you & rape your kids LOL",

but a world of difference from

"If Girls Aloud don't play Biology I'm gonna nuke the site from orbit!"

As for saying you would celebrate if some celebrity died, I would say the threshold for "appalling behaviour would be reached if you included their @twittername so that they saw it in their feed.