Maybe Kirk was afraid, Mitchell would ultimately become so powerful that he could force starships to come to him regardless of a quarantine zone ... or escape from Delta Vega some other way.
Or Kirk just had "issues" after Mitchell killed Kelso.
I´ve always wondered, if Mitchell was killed at all. When that rock fell on him he had already regained his power. And with his power present, not even a blast from a phaser rifle could harm him.
Just a thought ...