Where No Man Has Gone Before - S1-E4
Corrected entry: If the point was to repair the ship, and maroon Gary Mitchell, then why have Lee Kelso wire a destruct button, thereby threatening Gary's life and placing Lee's life in jeopardy when Gary's powers could overcome him? If they were afraid Gary would become too powerful left on his own, then marooning him should not have been an option.
Correction: It's explained in the episode that the plan is to maroon Mitchell there, but Kirk wants a backup in case they aren't able to escape or Mitchell becomes too powerful.