Write a scene in which your protagonist discovers something that prompts him/her to imagine a colleague is a psychopathic murderer.

It was after work one evening when Mark discovered something that made him imagine his colleague was a psychopathic murderer. He was walking through the parking lot to his car, when he saw his colleague’s car parked next to his. In the backseat of the car, he saw what looked like a bloody knife. His first instinct was to call the police, but something made him hesitate.

He thought about how his colleague had always been a little off, always making jokes about killing people. At first, he had thought it was just a dark sense of humor. But now, he wasn’t so sure. What if his colleague was actually a psychopathic killer?

He decided to watch him more closely, to see if he could catch him in the act. He would have to be careful though, because if his colleague found out he was watching him, he could be in danger.