Matt Decker

Commodore Matt Decker was a Human Starfleet flag officer assigned as the commander of the USS Constellation. 
In 2267, the Constellation was on a routine survey mission near System L-374 when it encountered an alien doomsday machine systematically destroying the planets of the system. 
Decker remained on board to confront the machine, beaming his entire crew to what he believed was safety. When the alien machine destroyed that planet, and crippled the Constellation, Decker was the only survivor. 
Decker was later discovered by the USS Enterprise, who transported him from his broken ship. Consumed with grief over his actions, and with an almost reckless desire for revenge, Decker seized command of the Enterprise after losing contact with the Constellation, leaving Captain Kirk stranded aboard the Constellation. 
He then launched an attack on the planet killer, a futile move which resulted in severe damage to the Enterprise. When communications were restored with the Constellation, Captain Kirk gave Commander Spock a direct order to relieve Decker on his personal authority as Captain of the Enterprise. Relieved of command by Spock, Decker subsequently stole an Enterprise shuttlecraft, piloting it into the maw of the creature in an attempt to destroy it. 
Decker was killed in the attempt. While the attack failed to cause significant damage - the Planet Killer had lost a small amount of power, demonstrating to the Enterprise crew how to disable the creature. Kirk drove the Constellation into the maw of the Planet Killer, which died when it was unable to handle the energy released by the Constellation's exploding impulse engines. Following the battle, Captain Kirk noted in his log that Decker gave his life in the line of duty.