There is no way to know for certain. In general, it's a good idea to assume that they will, and find some way to get away from them or otherwise protect yourself.
I agree with the above poster. It's one thing to talk , but it's another to do.
Narcissists are no different in many ways than plain old abusers, so take this person at their word and watch your back. If they phone you with threats ALWAYS answer your home phone on "speaker phone" and tape the call from the narcissist. This is proof for the police. Once you have to the proof cease to have any contact with the narcissist and don't pick up that phone! It's adviseable you seek legal counsel and also report this to the police (make a copy of that tape.) When someone threatens your life take it very seriously!
TIPS: Don't park in under-ground parking. When coming out of a building at night (especially malls) leave with a group of people and go straight to your car. If you are leaving a grocery store in the evening ask someone to help you out with your groceries (they will give this service.) PEPPER SPRAY is legal! Carry it with you and when you are away from home (walking alone or in the dark) snap the trigger on and carry it! If you jog or go for walks go with a friend. If you feel even a little uncomfortable staying in your home, then stay with your parents, relatives or a friend. Listen to your instincts! ALWAYS be aware of your surroundings.