A Deathtweet is when someone uses a twitter account to make death threats to someone else. This is typically done using a fake account so the DeathTweet is not linked to their personal account if they have one essentially allowing the tweeter to remain anonymous. Although it is sometimes preceded by a DeathTweet a Deadtweet is not to be confused with a DeathTweet as it is an announcement of someones death.
Oh damn, did you see Glenn Becks page after the teaparty rally? He got like 80 deathtweets in a matter of hours!