I agree with what the Cat said. First amendment freedom of speech and its exceptions are worth a book by themselves.
Whether this particular speech is protected by first amendment is hard to say. As we often say in law, it depends. Whether a speech is protected is often a question for a trier of fact. A person may be prosecuted for various conducts and raise first amendment freedom of speech as a defense. It depends on who can persuade the judge.
As we all know, nothing is absolute. Freedom of speech is not absolute. Over the years, the Supreme Court excluded a series of speeches from the first amendment protection. For example, child pornography, defamation, fighting words, perjury, obscenity, true threats, among others.
This particular speech, if anything, could be put under the category. My personal opinion is that it is not a true threat and is still under the protection of freedom of speech. Just like somebody says, I want to kill the president. It is not a crime and FBI is not that stupid to investigate such a thing. However, there is a limit. Although the high Court never had a bright line rule on what is a true threat, lower courts developed a theory to distinct a true threat from a protected speech. For example, if the speech was given after a heated discussion, in front of an audience, can incite immediate violence, or it was repeated, it could be ruled true threats, especially after 911.
In your particular example, it was given in internet forum. Nobody knows how many audiences there were and what their reaction would be. It seemed like a personal thing that would not arouse immediate action in the audience to breach peace. Also, it includes choices: if possible, which means the speacher may not take immediate action because killing a person may not be possible due to various reasons. It violates law at least. That is why I said in my opinion this is not an unprotected speech.
That being said, there is very little background information on how and when this speech was given. For example, if there was a pattern of stalking, or the parties engaged in physical contact before, or the person giving the speech in attempt to solicit others to actually take actions, etc., this would change the entire analysis.
