This scenario I made up has been bothering me for years and I want to know what others think of it. It will probably sound pointless to consider to most people, but this type of thing interests me.

Imagine that you're about to be murdered by someone (after being robbed or raped). He tells you that he needs to kill you so that you don't report him to the authorities, since you've seen his face and will be able to identify him. The only way he won't kill you is if 'promise' not to tell anyone about him. He then tells you that he's undergone training which has resulted in him having the ability to determine conclusively whether or not someone is lying from his/her facial expression (pretend you're 100% sure he can really do this to anyone). Do you think it would be possible to make the decision not to tell anyone?

The problem is, if you successfully tell him honestly that you aren't going to tell anyone, after he's gone, you're no longer threatened by him and have nothing keeping you from telling the authorities. But if you do tell the authorities at that point, you should have realized that you'd come to that decision back while you were being threatened, and therefore would have been lying when you told him you weren't going to tell anyone.