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)

As observed by the Third Doctor, the Ogrons served as mercenaries doing the "dirty work" of other species, most notably the Daleks. (TV: Day of the Daleks, Frontier in Space)

The Tenth Doctor took the Judoon to be a mercenary police force. (TV: Smith and Jones) The Thirteenth Doctor identified them as intergalactic mercenaries. (TV: Fugitive of the Judoon)

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)

Kane acquired an army of mercenaries to help him get revenge on Proamon. (TV: Dragonfire)

In an introduction that wrote for Doctor Who Magazine, he admitted that his life as the megalomaniac's megalomaniac wasn't all "murder, miniaturisation, and mercenary acts", and did have acts of so-called "charity work", which often involved more murder. (PROSE: A letter from the Master)