In addition to have sinned against his religion, Robertson is guilty of crimes which are considered acts of terrorism. He has violated both US federal laws and a number of international laws. He should be arrested and charged now. Death threats are considered an abuse of free speech rights. To threaten some one with death is terrorism, by definition.
Sadly, it looks as if he will get away with it, after all he was only expressing a US policy towards the heads of state of other countries that as been in effect for over 200 years.
See the attached file for legal information about the laws he has violated.