Should a joke about killing the president be protected by the right to free speech and the First Amendment? Or a poll on Facebook asking if Obama should be assassinated? Or a rap song about “killing a cop”? Or do such things cross a line beyond which the government can intervene, can limit the freedom of speech of those involved, and can punish them for having committed a crime? I would say: it depends.
In US jurisdiction, the Brandenburg v. Ohio case stipulates that abstract advocacy of violence is protected speech under the First Amendment. However, it is equally acceptable, also according to Brandenburg v. Ohio, that speech which incites imminent, illegal conduct – including violence – may itself be made illegal:
The constitutional guarantees of free speech and free press do not permit a State to forbid or proscribe advocacy of the use of force or of law violation except where such advocacy is directed to inciting or producing imminent lawless action and is likely to incite or produce such action.
So, advocacy of violence can only be prohibited when there is clear incitement of an imminent violent act, as well as the likelihood that this incitement produces or helps to produce such an act.
In the specific case of death threats, the Supreme Court case is Watts v. United States (1969). There it says that only true threats aren’t constitutionally protected; mere hyperbole, humor or offensive methods of stating political opposition are protected. What is a “true threat? According to Virginia v. Black (2003),
a statement can’t be a punishable threat unless it’s made “with the intent of placing the victim in fear of bodily harm or death.” Thus, following Black, a statement is a punishable threat only if a reasonable listener would understand it as a threat of attack and the speaker intended that the listener get that impression. (source)
Personally, I wouldn’t place too much weight on the second clause in that last sentence (after the “and”). I think it’s sufficient that the listener gets the impression of a threat and that the threat produces reasonable fear, even when the person stating the threat didn’t really mean it and was just joking (hence no real “intent”). So a joke about a bomb while on an airplane shouldn’t be protected, while a joke on the radio about killing the president should be protected, because the president or anyone else would probably not take it very seriously. The context of the threat is important. Even when there is clear intent and therefore not just a joke, but no likelihood of the threat being carried out, I would also propose to protect freedom of expression. The main focus is on the reaction of the reasonable recipient and the risk to which he or she is exposed (this focus contains a subjective and a factual element: perception/reaction and factual risk).