Mercenary

A mercenary was a soldier who took part in conflicts for private gain. General Grugger led a group of Gaztak mercenaries who worked for Meglos. (TV: Meglos)

The Tenth Doctor took the Judoon to be a mercenary police force. (TV: Smith and Jones)

After the destruction of the Tenth Sontaran Battle Fleet and his defeat on Earth by Sarah Jane Smith, the disgraced Commander Kaagh worked as a mercenary for the Bane. (TV: Enemy of the Bane)