If someone causally or even affectionately exclaims "I'll kill you for that!" I understand it's not likely illegal.
But if it includes intent to perform or even incite I would think that they would be investigated.
I think that is where it gets murky pretty quickly. I know they can't argue affection here, but I'm not sure that everybody who says what they would do if they saw someone is going to get charged for it.
__________________
There are no weapons of mass destruction in Iraq, Chuck Norris lives in Houston.
Either the United States will destroy ignorance, or ignorance will destroy the United States. – W.E.B. DuBois