Does being a religion give you a license to say anything you like?
We have laws against hate speech even though we have free speech in general, and we have laws against speech that encourage a specific crime.
The US does not have laws against hate speech. The article you linked to explains that.
We guarantee freedom of religion, but we also guarantee freedom of life. Which one has priority?
I acknowledge your intent here: Islam calls for the deaths of many kinds of people. But religion and life are not in conflict. Be careful: that is a false dichotomy and a dangerous generalization.
Maybe we should say categorically that you *can't* preach that it's OK to kill people of a certain class, whatever the class might be.
Hmmm... Now this is interesting... let us think it through. It sounds like you propose some kind of criminal penalty for a religious group to call for people to be killed. Does this affect only groups, or individuals? What about secular people who do the same? Should it become illegal to threaten someone in general?
Threatening someone with harm, when you show capability and intent to carry out that harm, is called assault. Assault is a crime in the United States. Assault is defined carefully, because really, how many people have called for the death of celebrities or politicians? Or call for the death of immigrants? Or certain classes of criminals? The average Joe calls for the death of lawyers on a daily basis. ;-) Perhaps it should be illegal to call for the death of any group of people?
Implementing this would be hard. Would we round-up religious leaders who call for the death of gays? I'm not sure how many of them are really living in the US anyway. You cited Westboro, which is a good example, but they haven't actually killed anyone... hmmm.... I suspect we could round-up the Westboro folks on assault already since they have carried out a number of their threats, but so far just protests. Seems like they would have a good chance of winning such a case. Although it would certainly send a message.
Suppose we did round-up such people: would it help, or would it merely cause the crazies to lash out? There is a thought that by allowing racist nutjobs like the Nazis and the KKK to go about their business in public, they demonstrate that they are crazy, and actually limit the growth of their own organizations. Some feel that by banning such things, they go underground where they are not publicly criticized and can quietly proliferate. There is a real fear of that kind of thing in Germany.
This becomes a slippery slope, which is why the founders of the United States wrote the first amendment.