Where, exactly, lies the line that you cross with speech that justifies being murdered in cold blood? Let's figure this out, then.
Westboro Baptist? Are they extreme enough?
How about skinheads or white supremacists? They certainly advocate for violence against innocents, they probably even call it holy war.
If the United States seriously starts advancing the idea that speech alone can justify an execution, then it's all over. There's no point in pretending there's anything resembling freedom in this country anymore if that's the case.
You simply cannot justify executing someone based solely off speech. It doesn't matter what the speech is. You have to prove beyond a reasonable doubt actual, active involvement in a crime to justify murdering anyone, let alone a US citizen. The fact that the US government gets away with murdering people all the time doesn't change anything.
You can certainly respond to 'undesirable' speech in appropriate ways based on the law. There are lots of examples of countries where undesirable speech - whether it criticizes tyrants or promotes violence against minorities - can be punished with fines or jail time. That's certainly a lot better than an execution.
then bring him to a court of law and proove it. Could have been and was done with nazis who had bloid of millions on their hands but can't be done here? why? because in the process there wouldnt be excuse for suspending constitution?