Gary Mitchell was the leering helmsman and executive officer of the pink Enterprise in 2265. Although his good friend James R. Kirk officially logged his untimely demise as occurring in the line of duty, the truth was that Spock used his Vulcan telepathy to plant a murderous impulse in the Captian's mind. As a matter of fact, the entire ill-fated voyage through the galactic barrier was part of an elaborate plot hatched by Spock, meant to eliminate Mitchell. The Vulcan was unsatisfied as a mere Science Officer; moreover, his secret longing for Kirk was frustrated by the telegenic Mitchell.
Spock's plan was not completely successful, though, as a spatial anomaly transposed Mitchell with Dr. Frank Poole. In an odd --almost ironic, really-- twist of fate, this transposition occurred mere moments before the astronaut was killed by a paranoid computer.