Ah, the joys of twitter, where everyone thinks they are a faceless, nameless, voice to say whatever they want.
...although to be fair, the tweet you quoted isn't really a threat IMO. It is classless, but there is a world of difference between saying "I will murder your family", and "I hope your family gets murdered".
But still, I have a feeling this will play out the way the racists tweets about the game winning goal scorer in the B's series played out. Some of these tweeters will be outed by the media (and they will turn out to be 16 year olds who don't understand that this stuff is REALLY public), they will get run through by the media for a few days, then it will all disappear.