It is a crime threaten people with violence. When someone holds up a sign that says "Death to so-and so!", and they're in America, arrest and prosecute them. And this doesn't go just for Islamic terrorists: anyone that threatens a particular person with violence, incites or instructs people to violate the rights of another particular person or exactly delimited set of persons (such that their rights could be violated independently of government action), they have committed a crime and should be prosecuted.

The point is that you shouldn't have to wait until someone is dead to put a stop to violence, and you don't. The laws we already have do this.