The line between trolling and threatening...
The line between the two is fine, but once something life-threatening like an explosive device gets involved, it leans towards the threatening.
Saying "Oh I wish someone blew you up" is more trollish than "A bomb's been placed outside your house, muahahaha."
And people, after all this time and all these Twitter/Facebook cases, still don't get that there is no such thing as anonymity... Even when online you'll have to take responsibility for what you write.