Genocide

Genocide was the destruction or attempted destruction of either an entire species or specific group of people. The Tenth Doctor claimed that, if one looked up the word genocide in a dictionary, they would see a picture of him, with the caption "over my dead body." (TV: The Doctor's Daughter)

Genocide can be contrasted with murder, which involved the deaths of individuals, rather than entire populations.

Human genocides

 * When Brigadier Lethbridge-Stewart destroyed the Wenley Moor Silurian colony, the Third Doctor was disgusted by the act, considering it genocide. (TV: Doctor Who and the Silurians)
 * Bennett believed that he had wiped out the Didonians. However, at least two had survived. (TV: The Rescue)
 * Humans may have committed near-genocide against the Wirrn in Andromeda when they destroyed their Breeding Colonies. (TV: The Ark in Space)
 * The Fourth Doctor helped human scientists kill the last of the Pescaton by the application of high-frequency sound waves. The Pescatons had fled their dying homeworld in search of another planet with large areas of salt water that could support them. (AUDIO: Doctor Who and the Pescatons)
 * The Eleventh Doctor accused the pirate Solomon of genocide after finding out he ejected the Silurians on-board their Ark into space. (TV: Dinosaurs on a Spaceship)

Dalek genocides

 * As the Daleks believed in racial purity and their superiority, they were responsible for many acts of genocide during their history. Races they have destroyed include: Koral's species (PROSE: Prisoner of the Daleks) and Eve's species. (TV: The Mad Woman in the Attic)
 * The Daleks wiped out the last Mechonoids on Magella (although considering that the Mechonoids were robots and not technically a living species, whether this counts as genocide is debatable). (PROSE: Birth of a Legend)
 * Dalek Caan committed genocide by killing all the Dalek-humans when they refused to obey the Daleks. When the Doctor confronted him, he offered help as he had already seen one genocide and had no wish to cause another. (TV: Evolution of the Daleks)

Doctor genocides

 * The Fourth Doctor destroyed Sutekh, the last known Osirian, finishing the genocide of the Osirans. The Doctor even taunted Sutekh as he died in the "temporal trap" - asking him how long Osirians live. 1,000 years? 10,000 years? Apparently, about 7,000 years. The Doctor's final taunt to Sutekh was, "The time of the Osirans is long past. Go." (TV: Pyramids of Mars)
 * The Fourth Doctor killed the last Kastrian (TV: ''The Hand of Fear)
 * With Leela's help, the Fourth Doctor destroyed the last of the Fendahl, finishing the genocide of the Fendahl. The Doctor dropped the remains of the Fendahl into a supernova to ensure the destruction of the Fendahl forever. (TV: Image of the Fendahl)
 * The Fourth Doctor fulfilled a Time Lord's solemn duty by killing the King Vampire, the last of the Great Vampires and sole vampire survivor of the Vampire-Time Lord war. Thus ends the Time Lord's genocidal war against the Great Vampires. (TV: State of Decay)
 * When the Sixth Doctor was forced to kill the Vervoids to stop them killing the humans on Hyperion III, this was used against him in a Trial occurring in his own past. When the Valeyard accused the Doctor of genocide, the Doctor said that he had no other option. (TV: Terror of the Vervoids) When the Eighth Doctor travelled to the events of the trial and met his sixth incarnation, he dismissed the charge due to the artificial nature of the Vervoids' creation. (PROSE: The Eight Doctors)
 * The Doctor committed double genocide when he destroyed the Time Lords and most of the Daleks at the end of the Last Great Time War. (TV: The End of Time)
 * During the Dark Times, the Fledgling Empires all but wiped out the Racnoss to prevent their devouring of planets. The Tenth Doctor later made them extinct when he flooded a ship of surviving young within the Earth. He was unknowingly aided by, who ordered the destruction of the Webstar which resulted in the death of the Empress of the Racnoss. (TV: The Runaway Bride)
 * The Doctor accused his Meta-Crisis duplicate of genocide when he destroyed all the Daleks in the Medusa Cascade, along with their fleet and the Crucible. As he thought he was too dangerous to be left to his own devices, he left him in Pete's World with Rose Tyler so she could make him better as she did for the Ninth Doctor himself. (TV: Journey's End)

Other genocides

 * By the time of the Thousand Year War between the Kaleds and the Thals on Skaro, the Kaleds had wiped out the Dals and the Tharons, the two other sentient humanoid races who had evolved on the planet. (AUDIO: Purity)
 * The Bannermen attempted to bring about the extinction of the Chimerons. Only Delta escaped, along with an egg, and after Billy converted into a Chimeron, the species could continue. (TV: Delta and the Bannermen)
 * The Caxtarid wiped out four indigenous sentient species on Kapteyn 5 after their conquest of the planet. (PROSE: The Room With No Doors)
 * Rivesh Mantilax invented a genetic weapon that would allow him to destroy the entire Viperox race. The Doctor stopped him from using it, insisting that no one had the right to wipe out a whole species. However, he did modify the device to merely impede the Viperox, halting an attempted invasion of Earth and driving Lord Azlok and the aggressors back to Viperon. (TV: Dreamland)
 * After the death of all his species, Androvax went on a vengeful vendetta, destroying twelve planets before finally being arrested by the Judoon when he attempted to destroy Earth. (TV: Prisoner of the Judoon)
 * Miss Myers, a member of a fleshkind race, created a child, intending for her to exterminate the metalkind. This plot was foiled by Sarah Jane Smith. (TV: Sky)
 * When the Weeping Angels arrived on Alfava Metraxis, they wiped out the Aplans. (TV: The Time of Angels)

Planned genocides

 * The Shakri desired to destroy mankind because they considered human beings "pests," but were stopped by the Eleventh Doctor. (TV: The Power of Three)
 * The Voord on Kandalinga planned to wipe out the native Fishmen once they were no longer of use to them, but were stopped by the First Doctor. (PROSE: The Fishmen of Kandalinga)

Behind the scenes

 * According to the Decide Your Destiny book, The Corinthian Project, the Dax were almost wiped out by the Daleks. However, because the novel has multiple possible endings, it's difficult to assert with certainty that the event ever occurred.