Yup! the answer is Time.
This is one of the scarier riddles I know.
A man and a woman were in a car driving down the road when their car broke down. Seeing no other way out, the man decided to walk to the nearest town for help. He told his wife to lock the doors and stay inside the car, which she did. An hour later, the man returned, and found his wife dead, along with a stranger in the car. The wife stayed in the car the entire time and locked the doors, and there was no sign of a struggle. Who is the stranger, and what happened?