Why does Mr. Mead try to respond to the police car with "But-"?

The police car tries to hurt him.

The police car is after the wrong person.

Mr. Mead doesn't think he did anything wrong.

Mr. Mead doesn't understand what is happening.

Mr. Mead tries to respond with "But-" because he doesn't understand what is happening. He may feel confused or worried about why the police car is approaching him. It is a natural human response to try and express your confusion or seek clarification when faced with a situation you don't comprehend.