FYI - it's because he specifically said that he was going to stab them. That's a very real threat and actionable, most people have access to knives.
Now saying you're going to cut their heads off with a guillotine? That's protected by free speech, and you'd 100% be able to argue that in court because I guarantee you don't have a guillotine or know how to build one (hopefully, maybe you do - in that case do not say this).