Matt Decker was a Starfleet commodore whose ship and crew were lost to the planet killer, and who died trying to destroy the machine himself.
Matt Decker was a Starfleet commodore in command of the USS Constellation. In The Doomsday Machine, his ship was crippled by a giant planet-destroying machine. Decker beamed his crew down to a nearby planet for safety, but the machine destroyed that world, killing all of them while Decker remained helpless on his ship.
The Enterprise found the Constellation drifting with Decker alone aboard, deeply traumatized. While Kirk and Montgomery Scott stayed on the wreck, Decker was sent to the Enterprise and used his rank to take command, leading the ship into a hopeless attack on the machine. Spock finally relieved him of command, but Decker took a shuttlecraft and flew it straight into the machine's mouth. His death showed that an explosion inside the machine could hurt it, and Kirk destroyed it by sending the Constellation in with its engines set to blow.
Decker is a tragic figure of the original series, a captain broken by guilt whose last act helped save others.