The anti-gun freaks who promote "SWATting" are stating they intend for people carrying guns to die from their lies and hysteria. To me, this constitutes a legitimate threat. If someone were to pre-emptively kill one of them, I'd regard it as self defense.

Also, it fits the legal definition of conspiracy--discussion of intent to commit an illegal act, followed by at least one overt act (that need not be criminal itself, merely supportive) (BUT, has been felonious, removing all doubt).

So every member of their group who has made a post or statement to that effect is a felon, conspiring to commit murder.

And until they are arrested for such, it would be completely reasonable to kill them on contact.

This is called "escalation." It's also called "Responding to a credible threat."