We already have the basic framework in place - that can be applied. Freedom of religion IS protected, but threatening people, and then those threats being carried out, is not protected.

But that isn't the issue, the issue is how far and using what methods does the U.S. choose in order to stop mega-death. If a religious organization in the U.S. is preaching death to all Americans by any means, and no law enforcement agencies know of it, then one could (and I would) argue the government is failing in one of its primary missions, to protect Americans. How to do that protection is the hard part.