I saw it when I was a child - long time ago.
A couple were driving late at night, driver behind them following in his own vehicle but behaving strangely (they noticed). They start testing him - they stop, he stops, they pull over, he pulls over, they drive again, he drives again. May have repeated this a few times. Eventually they stop and pull over and the man in the couple gets out to confront. The strange driver from the other vehicle also gets out, and shoots him dead without saying anything. then Drives off. can't remember if he shot the woman also but probably not as she may have been the one recounting the story (I don't think there were any witnesses). Has always stuck with me and would love to find the ep again.