The First Amendment does protect that though. As long as the speech isn't intended to incite imminent lawless action, calling for murders is absolutely protected.
E.g. "We should go harm X" is arguably illegal, since it's an immediate call to action.
However, "It'd be great if X died" or "All Y should die" are certainly protected.
In the same vein, this is why "Punch a Nazi" is totally legal: assault is illegal, but you're not immediately inciting a lawless action. "Let's go punch that Nazi", less so.
The essential thing missing from all of these discussions (unless I am misunderstanding people) is the immediacy of the incitement. The context in which the speech matters. That is where the imminent and likely parts properly arise from.
If you write a book advocating for violence against x or y group or individuals that is permissible, but if you were in a crowded square and advocated the same thing when those targets were also in the square and it is likely that your incitement will lead to violence then it is not. That's incitement, it's imminent, and it is for a lawless act. But again, if you did it at home on your blog in some nebulous sense that isn't likely to cause some specific event then it is protected speech.
An important distinction here is that "true threats" are a separate category from what we are talking about. A true threat doesn't have a "likely" or "imminent" component and so is even broader in scope than violent speech in general. That is, true threats are not protected.
E.g. "We should go harm X" is arguably illegal, since it's an immediate call to action.
However, "It'd be great if X died" or "All Y should die" are certainly protected.
In the same vein, this is why "Punch a Nazi" is totally legal: assault is illegal, but you're not immediately inciting a lawless action. "Let's go punch that Nazi", less so.