Ronald Tracey
Ronald Tracey was a 23rd century Starfleet captain, the commanding officer of the Constitution-class starship, USS Exeter.
Captain Tracey led an Exeter landing party to the surface of planet Omega IV in 2268, were they inadvertently contracted the Omega IV virus, and passed it along to the rest of their crew. Tracey remained on the surface while his crew died painfully in orbit, and found that the atmosphere provided him immunity to the virus. In violation of the Prime Directive, Tracey struck a bargain with members of the planet's Kohm faction, and participated in their ongoing war against the Yangs. Newly armed with a few phasers, Tracey and his Kohm allies slaughtered thousands of Yangs.
The USS Enterprise eventually discovered Exeter and the dehydrated remains of its crew. The boarding party, led by Captain James T. Kirk, were immediately exposed to the virus and beamed down to the relative safety of the planet surface where they discovered Tracey. After his crimes were exposed, Tracey murdered Kirk's security guard, Lt. Galloway; subdued the landing party, and demanded a supply of phasers from Enterprise.
Tracey erroneously believed the Omega IV virus held the key for immortality. He considered it an opportunity more important than the Prime Directive, and if missed, "...a crime against all Humanity." After his failure to enlist Kirk in his cause, Tracey and the Enterprise officers were captured by the Yangs. In last-ditch effort to regain control of his fate, Tracey played on the Yang's superstitions and tried to convince them that Kirk and his officers were evil, and must die. The Yangs matched Tracey and Kirk in combat to the death, to decide the truth of Tracey's wild claims. The intervention of an Enterprise rescue party permitted Kirk to spare Tracey's life and place the once-respected starship Captain under arrest. (TOS: "The Omega Glory")
Background
Captain Tracey was played by actor Morgan Woodward.
Tracey is the only Original Series character, other than James Kirk, to wear a Captain's rank insignia and be seen (alive) as the Commanding Officer of a starship. All other active starship commanders were commodores (Matthew Decker, Robert Wesley, and Commodore Stocker albeit temporarily).
His fall from grace seems to contradict everything mentioned in Star Trek about the caliber that Starfleet places on its starship commanders and, given the years of experience that Tracey must have had under his belt, it seems unlikely that he would have so easily turned into a cold blooded killer while violating the Prime Directive. Nevertheless, it seems to have introduced a precedent that Starfleet captains are capable of such acts. Other starship captains shown having abandoned their principles as Starfleet officers include Benjamin Maxwell of the USS Phoenix, Erika Benteen of the USS Lakota, and Rudolph Ransom of the USS Equinox. To an extent, even Captain Benjamin Sisko could be considered to have lost sight of Federation ideology during his involvement with the assassination of Romulan Senator Vreenak.
Tracey's quest for immortality, in spite of a great moral cost, would later be echoed in Star Trek: Insurrection, when Vice Admiral Matthew Dougherty attempted to harness the metaphasic radiation unique to the Baku planet, in 2375.