The beast might have guessed his identity and wanted to show his public spirit by handing him over to the police.

The newspaper in his hand! Of course.

He had read his description there, and identified him.
Charles found himself conjecturing how the man would set about carrying out his task of public watchdog, if that was in his mind.

He pictured the possibility of him appealing to the others in the compartment.

He might get up and say: "There is a murderer in this compartment.