If that happens I would focus primarily on the threat to the safety and security of the flight itself, and less on making it your individual complaint. That takes it to a different level, and makes the purser directly responsible for dealing with it. Otherwise it sounds like just two seat mates who can't get along. Though you don't elaborate on the threat, if it was indeed something that made you in fear for your safety, I would demand to speak to either the captain or one of the air marshalls. If there is law enforcement aboard, they are required to deal with any threat or perceived threat. Basically, while keeping calm and professional (so they don't accuse you ogf being the problem), demand action be taken to resolve the matter then and there.