The Proconsul seemed to believe that if he kept the Enterprise
crew from leaving and reporting back to Starfleet and the Federation he was minimizing or eliminating word of his world's existence. But secretly he might simply have been afraid (not that he'd show it) of what Starfleet might do with knowledge of his planet's existence and how Merrick and Kirk's personnel had been treated. I'd hazard a guess that the Proconsul was projecting his own likely reaction (one of retaliation) onto Starfleet and the Federation. He didn't seem to realize that Starfleet would probably be a lot more interested in the planet if a class one ship like the Enterprise
disappeared in that area.
Yeah, and he could well have been trying to prove how big and tough he was and simply hadn't counted on Kirk's (and Spock's and McCoy's) resolve. He thought Merrick was a typical Earthman and ship commander and figured Kirk could be just as easily pushed around.
There was always something rather sleazy about the Proconsul. Wouldn't have surprised me to learn he'd been watching Kirk's liaison with the slave Drusilla.